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

[Matt Arsenault via llvm-commits]

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arsenm wrote:

```suggestion
# RUN: llc -mtriple=amdgpu9.00-amd-amdhsa -O0 -start-before=si-lower-sgpr-spills -stop-after=si-lower-wwm-copies -o %t %s
```

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