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

[Jay Foad via llvm-commits]

https://github.com/jayfoad approved this pull request.

Whole stack LGTM. Thanks for working on this! Have you measured any compile-time difference from this change?

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