[llvm] [LV] Extend FindLastIV to unsigned case (PR #141752)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> The other alternative is a strange asymmetry in calls to addReductionVar.

After thinking some more, I think this is the best compromise: I've updated the patch.

https://github.com/llvm/llvm-project/pull/141752