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

[Pranav Taneja via llvm-commits]

pranavTanejaAMD 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 

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