[llvm] SimplifyIndVar: teach widenLoopCompare about samesign (PR #125851)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

Duplicate of https://github.com/llvm/llvm-project/pull/125764, since GitHub seems to have hung on that PR.

https://github.com/llvm/llvm-project/pull/125851