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

[Christudasan Devadasan via llvm-commits]

cdevadas wrote:

At what stage in the codegen we introduce WWM_COPY with physregs? This pass was originally intended to lower the liverange split copies inserted during wwm-greedy invocation. The WWM_Copies are expected to have only virtual registers and it requires VRM to handle them. 
The scenario described in the description seems to be coming from elsewhere (somewhere early during ISel? or from another custom pass?). I am not sure this is the right place to handle them. Because this pass is scheduled right at the middle of RA and what will happen to the WWM_COPY instances at various passes until this point? 

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