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

[Luke Lau via llvm-commits]

https://github.com/lukel97 commented:

I think this makes sense to me. To make sure I'm understanding this correctly, the benefit of using the umax reduction is that it allows us to handle IVs whose range contains the signed min. Whereas the smax reduction allows to handle IVs whose range contains the unsigned min?

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