[llvm] [LV] Extend FindFirstIV to unsigned case (PR #146386)

[Florian Hahn via llvm-commits]

https://github.com/fhahn approved this pull request.

LGTM, thanks. 

Some touched comments should probably be updated to explain both increasing and decreasing cases, added some more comments inline

https://github.com/llvm/llvm-project/pull/146386