Type test `List<T> is Iterator<T>` that can never succeed is not optimized away

[neon-sunset]

Description

Given simple benchmark code

using BenchmarkDotNet.Attributes;

[DisassemblyDiagnoser]
public class EnumTest {
 readonly List<string> list = ["hello", "world", "foo", "bar"];

 [Benchmark]
 public string Last() => list.Last();
}

It appears that both Last() and TryGetLast() that it calls out to get inlined which is good.
However, the type test of List<string> is Iterator<string> does not get optimized away even though it can never succeed:

runtime/src/libraries/System.Linq/src/System/Linq/Last.cs

Line 73 in b579962

source is Iterator<TSource> iterator ? iterator.TryGetLast(out found) :

This results in the following codegen:

; Benchmarks.EnumTest.Last()
 push rbx
 sub rsp,30
 mov rbx,[rcx+8]
 test rbx,rbx
 je short M00_L03
 mov rdx,rbx
 mov rcx,offset MT_System.Linq.Enumerable+Iterator`1[[System.String, System.Private.CoreLib]]
 call qword ptr [7FFE83476910]; System.Runtime.CompilerServices.CastHelpers.IsInstanceOfClass(Void*, System.Object)
 test rax,rax
 jne short M00_L01
 lea r8,[rsp+28]
 mov rdx,rbx
 mov rcx,7FFE838AFB20
 call qword ptr [7FFE8382D980]
M00_L00:
 cmp byte ptr [rsp+28],0
 je short M00_L02
 add rsp,30
 pop rbx
 ret
M00_L01:
 lea rdx,[rsp+28]
 mov rcx,rax
 mov rax,[rax]
 mov rax,[rax+48]
 call qword ptr [rax+18]
 jmp short M00_L00
M00_L02:
 call qword ptr [7FFE8382CD38]
 int 3
M00_L03:
 mov ecx,11
 call qword ptr [7FFE8347F7B0]
 int 3
; Total bytes of code 114

While the expected codegen would probably look like:

; Benchmarks.EnumTest.Last()
 push rbx
 sub rsp,30
 mov rbx,[rcx+8]
 test rbx,rbx
 je short M00_L02
 lea r8,[rsp+28]
 mov rdx,rbx
 mov rcx,7FFE838AFB20
 call qword ptr [7FFE8382D980] ;; TryGetLastNonIterator
M00_L00:
 cmp byte ptr [rsp+28],0
 je short M00_L01
 add rsp,30
 pop rbx
 ret
M00_L01:
 call qword ptr [7FFE8382CD38]
 int 3
M00_L02:
 mov ecx,11
 call qword ptr [7FFE8347F7B0]
 int 3

Configuration

.NET SDK:
 Version: 9.0.100-rc.2.24423.15
 Commit: 6bf8f98ca5
 Workload version: 9.0.100-manifests.c7b42424
 MSBuild version: 17.12.0-preview-24422-01+314cf24ea

Runtime Environment:
 OS Name: Windows
 OS Version: 10.0.22631
 OS Platform: Windows
 RID: win-x64
 Base Path: C:\Program Files\dotnet\sdk\9.0.100-rc.2.24423.15\

Regression?

No

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