[llvm] SCEV: teach isImpliedViaOperations about samesign (PR #124270)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> Hi,
> 
> I bisected (what I think is) a miscompile back to this patch.
> 
> Reduced indvars reproducer: `opt -passes="indvars" bbi-103889.ll -S -o -`
> 
> The problem is that with this patch it changes the input `%add = add i16 %0, 2` to `%add = add nuw nsw i16 %1, 2` but we're iterating from 32760 to 32780, doing "ult" comparisons, so we do get signed wrap around in the add.
> 
> [bbi-103889.ll.gz](https://github.com/user-attachments/files/18730249/bbi-103889.ll.gz)

I think #126409 has a reduced test case. Currently verifying that this patch is buggy: will revert and investigate, as it's not clear what's wrong with the patch.

https://github.com/llvm/llvm-project/pull/124270