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

[Matt Arsenault via llvm-commits]

arsenm wrote:

I think we need to stop adding these extremely specific hooks that just identify problematic situations and hack around them in individual transforms. I think we need to move towards fully modeling the uniform and divergent CFGs in MachineBasicBlock, and use dedicated restricted pseudo-copies where there's a potential temporal divergence issue 

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