[llvm] [AMDGPU] Fix GCNUpwardRPTracker. (WIP) (PR #71186)

[Jay Foad via llvm-commits]

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

Looks OK to me. I have not understood every detail but you know this code much better than I do.

https://github.com/llvm/llvm-project/pull/71186