[llvm] [AMDGPU] Post-RA AGPR copy elimination pass (PR #153901)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm requested changes to this pull request.

I want to spend time looking at the testcase, this is too much infrastructure for what should be a simple issue 

https://github.com/llvm/llvm-project/pull/153901