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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > Whole stack LGTM. Thanks for working on this! Have you measured any compile-time difference from this change?
> 
> [compileTimeDiff.log](https://github.com/llvm/llvm-project/files/12595643/compileTimeDiff.log)
> 
> Here's the difference between compile time from both the original and modified builds on compiling tests from llvm-test-suite

So didn't really do anything for x86, which is expected. Ideally we would have GPU compute compile time tracking but we don't 

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