Loop condition `i != T.Length` bounds check not eliminated

[xtqqczze]

This is not a particularly common pattern, but it is a missed optimization all the same.

Consider:

 public static int Array_NE(byte[] src)
 { 
 int sum = 0;
 for (int i = 0; i != src.Length; i++)
 sum += src[i];
 return sum;
 }
 public static int Array_LT(byte[] src)
 { 
 int sum = 0;
 for (int i = 0; i < src.Length; i++)
 sum += src[i];
 return sum;
 }

A bounds check is eliminated for the second method but not the first.

Applies to T[], string, Span<T>, ReadOnlySpan<T>, etc.

https://csharp.godbolt.org/z/bKzrE9Wjq

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Issue Details

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Author:	xtqqczze
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-

[EgorBo]

cc @BruceForstall if this loop shape can be recognized as normal, monotonically increased one

[xtqqczze]

Also missed in the backward iteration case, I think there there is an existing issue though.

 public static int Array_Backward_GE(Byte[] src)
 { 
 int sum = 0;
 for (int i = src.Length - 1; i >= 0; i--)
 sum += src[i];
 return sum;
 }

[hughbe]

Also missed in the backward iteration case, I think there there is an existing issue though.

 public static int Array_GE(Byte[] src)
 { 
 int sum = 0;
 for (int i = src.Length - 1; i >= 0; i--)
 sum += src[i];
 return sum;
 }

This could be a common pattern in cases where people have copied things from c

[xtqqczze]

Also missed in the backward iteration case, I think there there is an existing issue though.

Found the existing issue: #9505.

[xtqqczze]

Backward iteration case for i != T.Length, very common in C:

 public static int Array_Backward_NE(Byte[] src)
 { 
 int sum = 0;
 int i = src.Length;
 while (i != 0)
 {
 sum += src[--i];
 }
 return sum;
 }