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

[Pranav Taneja via llvm-commits]

pranavTanejaAMD wrote:

Have updated this branch to retain 'available' in LiveRegUnits as it is while still updating SIFrameLowering (as suggested by @arsenm in [[LiveRegUnits] Enhanced the register liveness check](https://github.com/llvm/llvm-project/pull/66061)). I apologise for the history on this PR that got lost with this update. 

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