[llvm] SimplifyIndVar: teach widenLoopCompare about samesign (PR #125764)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> I think something the tests don't cover is that the RHS is sign extended rather than zero extended. (They use constant 1 where it makes no difference.)

I didn't want to change the gold tests, so I've added some additional tests. Let me know if this is fine.

https://github.com/llvm/llvm-project/pull/125764