[llvm] [AMDGPU] Fix speculative register pressure queries (PR #208574)

[Lucas Ramirez via llvm-commits]

https://github.com/lucas-rami approved this pull request.

This makes sense to me. 

I wonder whether we would not be better off splitting `GCNDownwardRPTracker` into "internal iterator" and "external iterator" versions in the future. I find the mixing of both confusing/error-prone. I am not sure whether that's easily feasible given how we currently use the class though.

https://github.com/llvm/llvm-project/pull/208574