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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Are you trying to identify branches whose condition is divergent according to UA, but is likely to be uniform in practice?

I think it's the opposite. It's divergent, and known never uniform in practice 

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