[llvm] [AMDGPU] Eliminate likely-spurious execz checks (PR #117567)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

I feel like this deserves separate representation from branch frequencies, but I'm not sure what it would be. And it would involve more threading work to get to the late backend pass 

https://github.com/llvm/llvm-project/pull/117567