Use local assertion prop to omit casts on returns

[SingleAccretion]

When inserting normalizing casts for returns in pre-order
morph, fgMorphCast was used, which did not account for
the fact that local assertion propagation could tell us
that the cast is in fact unnecessary. Fix this by calling
fgMorphTree instead.

Also some miscellaneous code cleanup.

The diffs are almost non-existent, but are there:

coreclr_tests.pmi.windows.x64.checked.mch:

Summary of Code Size diffs:
(Lower is better)
Total bytes of base: 1700
Total bytes of diff: 1655
Total bytes of delta: -45 (-2.65% of base)
Total relative delta: -0.68
diff is an improvement.
relative diff is an improvement.

Detail diffs

Top file improvements (bytes):
-3 : 80797.dasm (-2.59% of base)
-2 : 237399.dasm (-2.70% of base)
-2 : 237455.dasm (-2.70% of base)
-2 : 237376.dasm (-3.85% of base)
-2 : 237442.dasm (-3.85% of base)
-2 : 237389.dasm (-2.70% of base)
-2 : 237443.dasm (-3.85% of base)
-2 : 237456.dasm (-2.56% of base)
-2 : 237465.dasm (-2.70% of base)
-2 : 237393.dasm (-2.82% of base)
-2 : 237390.dasm (-2.56% of base)
-2 : 237403.dasm (-2.82% of base)
-2 : 237459.dasm (-2.82% of base)
-2 : 237466.dasm (-2.56% of base)
-2 : 237377.dasm (-3.85% of base)
-2 : 237400.dasm (-2.56% of base)
-2 : 237448.dasm (-3.85% of base)
-2 : 237382.dasm (-3.85% of base)
-2 : 237469.dasm (-2.82% of base)
-1 : 237394.dasm (-1.43% of base)
25 total files with Code Size differences (25 improved, 0 regressed), 0 unchanged.
Top method improvements (bytes):
-3 (-2.59% of base) : 80797.dasm - Repro:ReadUInt16():ushort
-2 (-2.70% of base) : 237399.dasm - ConvTest:test24():byte
-2 (-2.70% of base) : 237455.dasm - ConvTest:test14():byte
-2 (-3.85% of base) : 237376.dasm - ConvTest:test1():byte
-2 (-3.85% of base) : 237442.dasm - ConvTest:test1():byte
-2 (-2.70% of base) : 237389.dasm - ConvTest:test14():byte
-2 (-3.85% of base) : 237443.dasm - ConvTest:test2():short
-2 (-2.56% of base) : 237456.dasm - ConvTest:test15():short
-2 (-2.70% of base) : 237465.dasm - ConvTest:test24():byte
-2 (-2.82% of base) : 237393.dasm - ConvTest:test18():ubyte
-2 (-2.56% of base) : 237390.dasm - ConvTest:test15():short
-2 (-2.82% of base) : 237403.dasm - ConvTest:test28():ubyte
-2 (-2.82% of base) : 237459.dasm - ConvTest:test18():ubyte
-2 (-2.56% of base) : 237466.dasm - ConvTest:test25():short
-2 (-3.85% of base) : 237377.dasm - ConvTest:test2():short
-2 (-2.56% of base) : 237400.dasm - ConvTest:test25():short
-2 (-3.85% of base) : 237448.dasm - ConvTest:test7():ubyte
-2 (-3.85% of base) : 237382.dasm - ConvTest:test7():ubyte
-2 (-2.82% of base) : 237469.dasm - ConvTest:test28():ubyte
-1 (-1.43% of base) : 237394.dasm - ConvTest:test19():ushort
Top method improvements (percentages):
-2 (-3.85% of base) : 237376.dasm - ConvTest:test1():byte
-2 (-3.85% of base) : 237442.dasm - ConvTest:test1():byte
-2 (-3.85% of base) : 237443.dasm - ConvTest:test2():short
-2 (-3.85% of base) : 237377.dasm - ConvTest:test2():short
-2 (-3.85% of base) : 237448.dasm - ConvTest:test7():ubyte
-2 (-3.85% of base) : 237382.dasm - ConvTest:test7():ubyte
-2 (-2.82% of base) : 237393.dasm - ConvTest:test18():ubyte
-2 (-2.82% of base) : 237403.dasm - ConvTest:test28():ubyte
-2 (-2.82% of base) : 237459.dasm - ConvTest:test18():ubyte
-2 (-2.82% of base) : 237469.dasm - ConvTest:test28():ubyte
-2 (-2.70% of base) : 237399.dasm - ConvTest:test24():byte
-2 (-2.70% of base) : 237455.dasm - ConvTest:test14():byte
-2 (-2.70% of base) : 237389.dasm - ConvTest:test14():byte
-2 (-2.70% of base) : 237465.dasm - ConvTest:test24():byte
-3 (-2.59% of base) : 80797.dasm - Repro:ReadUInt16():ushort
-2 (-2.56% of base) : 237456.dasm - ConvTest:test15():short
-2 (-2.56% of base) : 237390.dasm - ConvTest:test15():short
-2 (-2.56% of base) : 237466.dasm - ConvTest:test25():short
-2 (-2.56% of base) : 237400.dasm - ConvTest:test25():short
-1 (-2.00% of base) : 237383.dasm - ConvTest:test8():ushort
25 total methods with Code Size differences (25 improved, 0 regressed), 0 unchanged.

---

libraries.crossgen2.windows.x64.checked.mch:

Summary of Code Size diffs:
(Lower is better)
Total bytes of base: 521
Total bytes of diff: 506
Total bytes of delta: -15 (-2.88% of base)
Total relative delta: -0.17
diff is an improvement.
relative diff is an improvement.

Detail diffs

Top file improvements (bytes):
-4 : 131557.dasm (-5.48% of base)
-3 : 131554.dasm (-4.23% of base)
-3 : 131553.dasm (-4.23% of base)
-3 : 90249.dasm (-1.50% of base)
-2 : 33831.dasm (-1.89% of base)
5 total files with Code Size differences (5 improved, 0 regressed), 0 unchanged.
Top method improvements (bytes):
-4 (-5.48% of base) : 131557.dasm - Microsoft.VisualBasic.CompilerServices.VB6File:GetShort(long):short:this
-3 (-4.23% of base) : 131554.dasm - Microsoft.VisualBasic.CompilerServices.VB6File:GetByte(long):ubyte:this
-3 (-4.23% of base) : 131553.dasm - Microsoft.VisualBasic.CompilerServices.VB6File:GetChar(long):ushort:this
-3 (-1.50% of base) : 90249.dasm - Newtonsoft.Json.JsonTextReader:ConvertUnicode(bool):ushort:this
-2 (-1.89% of base) : 33831.dasm - Microsoft.CodeAnalysis.ArrayBuilderExtensions:Pop(Microsoft.CodeAnalysis.ArrayBuilder`1[Microsoft.CodeAnalysis.SyntaxNode+TwoEnumeratorListStack+Which]):ubyte
Top method improvements (percentages):
-4 (-5.48% of base) : 131557.dasm - Microsoft.VisualBasic.CompilerServices.VB6File:GetShort(long):short:this
-3 (-4.23% of base) : 131554.dasm - Microsoft.VisualBasic.CompilerServices.VB6File:GetByte(long):ubyte:this
-3 (-4.23% of base) : 131553.dasm - Microsoft.VisualBasic.CompilerServices.VB6File:GetChar(long):ushort:this
-2 (-1.89% of base) : 33831.dasm - Microsoft.CodeAnalysis.ArrayBuilderExtensions:Pop(Microsoft.CodeAnalysis.ArrayBuilder`1[Microsoft.CodeAnalysis.SyntaxNode+TwoEnumeratorListStack+Which]):ubyte
-3 (-1.50% of base) : 90249.dasm - Newtonsoft.Json.JsonTextReader:ConvertUnicode(bool):ushort:this
5 total methods with Code Size differences (5 improved, 0 regressed), 0 unchanged.

---

libraries.pmi.windows.x64.checked.mch:

Summary of Code Size diffs:
(Lower is better)
Total bytes of base: 348
Total bytes of diff: 341
Total bytes of delta: -7 (-2.01% of base)
Total relative delta: -0.08
diff is an improvement.
relative diff is an improvement.

Detail diffs

Top file improvements (bytes):
-3 : 105662.dasm (-1.32% of base)
-2 : 76226.dasm (-3.33% of base)
-2 : 76225.dasm (-3.33% of base)
3 total files with Code Size differences (3 improved, 0 regressed), 0 unchanged.
Top method improvements (bytes):
-3 (-1.32% of base) : 105662.dasm - Newtonsoft.Json.JsonTextReader:ConvertUnicode(bool):ushort:this
-2 (-3.33% of base) : 76226.dasm - Microsoft.CodeAnalysis.ArrayBuilderExtensions:Pop(Microsoft.CodeAnalysis.ArrayBuilder`1[Int16]):short
-2 (-3.33% of base) : 76225.dasm - Microsoft.CodeAnalysis.ArrayBuilderExtensions:Pop(Microsoft.CodeAnalysis.ArrayBuilder`1[Byte]):ubyte
Top method improvements (percentages):
-2 (-3.33% of base) : 76226.dasm - Microsoft.CodeAnalysis.ArrayBuilderExtensions:Pop(Microsoft.CodeAnalysis.ArrayBuilder`1[Int16]):short
-2 (-3.33% of base) : 76225.dasm - Microsoft.CodeAnalysis.ArrayBuilderExtensions:Pop(Microsoft.CodeAnalysis.ArrayBuilder`1[Byte]):ubyte
-3 (-1.32% of base) : 105662.dasm - Newtonsoft.Json.JsonTextReader:ConvertUnicode(bool):ushort:this
3 total methods with Code Size differences (3 improved, 0 regressed), 0 unchanged.

---

libraries_tests.pmi.windows.x64.checked.mch:

Summary of Code Size diffs:
(Lower is better)
Total bytes of base: 732
Total bytes of diff: 718
Total bytes of delta: -14 (-1.91% of base)
Total relative delta: -0.15
diff is an improvement.
relative diff is an improvement.

Detail diffs

Top file improvements (bytes):
-4 : 128650.dasm (-1.95% of base)
-3 : 128649.dasm (-1.48% of base)
-2 : 11111.dasm (-3.33% of base)
-2 : 11110.dasm (-3.33% of base)
-1 : 118419.dasm (-2.27% of base)
-1 : 269986.dasm (-1.27% of base)
-1 : 269988.dasm (-1.23% of base)
7 total files with Code Size differences (7 improved, 0 regressed), 0 unchanged.
Top method improvements (bytes):
-4 (-1.95% of base) : 128650.dasm - Microsoft.Diagnostics.Runtime.Linux.Reader:Read(byref):short:this
-3 (-1.48% of base) : 128649.dasm - Microsoft.Diagnostics.Runtime.Linux.Reader:Read(byref):ubyte:this
-2 (-3.33% of base) : 11111.dasm - Microsoft.CodeAnalysis.ArrayBuilderExtensions:Pop(Microsoft.CodeAnalysis.PooledObjects.ArrayBuilder`1[Int16]):short
-2 (-3.33% of base) : 11110.dasm - Microsoft.CodeAnalysis.ArrayBuilderExtensions:Pop(Microsoft.CodeAnalysis.PooledObjects.ArrayBuilder`1[Byte]):ubyte
-1 (-2.27% of base) : 118419.dasm - Microsoft.Build.Shared.LanguageParser.TokenCharReader:SinkCharacter():ushort:this
-1 (-1.27% of base) : 269986.dasm - Stashbox.Utils.Data.Stack`1[Byte][System.Byte]:Pop():ubyte:this
-1 (-1.23% of base) : 269988.dasm - Stashbox.Utils.Data.Stack`1[Int16][System.Int16]:Pop():short:this
Top method improvements (percentages):
-2 (-3.33% of base) : 11111.dasm - Microsoft.CodeAnalysis.ArrayBuilderExtensions:Pop(Microsoft.CodeAnalysis.PooledObjects.ArrayBuilder`1[Int16]):short
-2 (-3.33% of base) : 11110.dasm - Microsoft.CodeAnalysis.ArrayBuilderExtensions:Pop(Microsoft.CodeAnalysis.PooledObjects.ArrayBuilder`1[Byte]):ubyte
-1 (-2.27% of base) : 118419.dasm - Microsoft.Build.Shared.LanguageParser.TokenCharReader:SinkCharacter():ushort:this
-4 (-1.95% of base) : 128650.dasm - Microsoft.Diagnostics.Runtime.Linux.Reader:Read(byref):short:this
-3 (-1.48% of base) : 128649.dasm - Microsoft.Diagnostics.Runtime.Linux.Reader:Read(byref):ubyte:this
-1 (-1.27% of base) : 269986.dasm - Stashbox.Utils.Data.Stack`1[Byte][System.Byte]:Pop():ubyte:this
-1 (-1.23% of base) : 269988.dasm - Stashbox.Utils.Data.Stack`1[Int16][System.Int16]:Pop():short:this
7 total methods with Code Size differences (7 improved, 0 regressed), 0 unchanged.

---

Example of the expected improvement, from Microsoft.CodeAnalysis.ArrayBuilderExtensions:Pop:

G_M1359_IG01:
push rdi
push rsi
sub rsp, 40
mov rsi, rcx
G_M1359_IG02:
mov rcx, gword ptr [rsi+8]
mov rdx, rcx
mov edx, dword ptr [rdx+16]
dec edx
call [Builder[Int16][System.Int16]:get_Item(int):short:this]
movsx rdi, ax
mov rcx, gword ptr [rsi+8]
mov rdx, rcx
mov edx, dword ptr [rdx+16]
dec edx
call [Builder[Int16][System.Int16]:RemoveAt(int):this]
- movsx rax, di+ mov eax, edi
G_M1359_IG03:
add rsp, 40
pop rsi
pop rdi
ret

[kunalspathak]

LGTM