[llvm] AMDGPU: Fix temporal divergence introduced by machine-sink and performance regression introduced by D155343 (PR #67456)

[Nicolai Hähnle via llvm-commits]

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

Okay, I see now that we do need an explicit check for the block prologue for post-RA sinking. So I think this change should be good to go.

https://github.com/llvm/llvm-project/pull/67456