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

[Kiroo via llvm-commits]

zlfn wrote:

@arsenm I agree that this is not a fundamental fix in the long term. The proper direction would be to stop adding `implicit-def $superreg` and let next passes determine liveness from sub-register defs. Further down the road, replacing LiveVariables with LiveIntervals entirely, as you mentioned.

But LiveVariables does have bugs today: I fixed (1) a mistmatch between the code's comments and its actual implementation, and (2) a latent bug in `FindLastPartialDef`.

Merging this PR shouldn't conflict with any future structural changes. I'd suggest racking deeper issue as a seperate issue.

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