[llvm] Eliminating li of 0 into arg registers of unused arguments (PR #122741)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> If I can no longer use this function, the patch will become quite large. Can I still use getUNDEF in this patch while keeping the undef deprecator, and then introduce a follow-up patch to remove getUNDEF later?
> 
> @hubert-reinterpretcast , @topperc

Ignore the undef deprecator, it is broken. It should not be complaining about getUNDEF (or really anything in codegen)

https://github.com/llvm/llvm-project/pull/122741