[llvm] [AMDGPU] Preserve physical WWM_COPY destinations (PR #227279)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> At what stage in the codegen we introduce WWM_COPY with physregs?

I assume we would need this to implement SGPR argument passing packed in VGPR lanes, eventually 

https://github.com/llvm/llvm-project/pull/227279