[llvm] [CodeGen] Fix partial def handling in HandlePhysRegUse and FindLastPartialDef (PR #186437)

[Kiroo via llvm-commits]

zlfn wrote:

@shiltian @jayfoad, I saw your names on a related PRs. Would you be able to review this one as well? I suspect this bug may also affect AMDGPU, but I'm not familiar enough with that target to test it myself.

https://github.com/llvm/llvm-project/pull/186437