[llvm] [AMDGPU] [SIFrameLowering] Use LiveRegUnits instead of LivePhysRegs (PR #65962)

[Pranav Taneja via llvm-commits]

pranavTanejaAMD wrote:

> FYI you're currently only supposed to push one commit per pull request
I did want to follow the suggested "one commit per PR" as prescribed in the policy but these commits are actually two separate related changes and since stacked reviews are not yet supported here, creating a monolithic review for such a code change didn't seem ideal.   
Would you have any suggestions on dealing with this currently? 


https://github.com/llvm/llvm-project/pull/65962