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

[David Spickett via llvm-commits]

DavidSpickett wrote:

Not sure there even is an AVR subscriber/review team, so I've added some people I found in the git logs.

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