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

[Fabian Ritter via llvm-commits]

ritter-x2a wrote:

@jayfoad The idea is to be more optimistic than that, in the realms of "does the condition use bits that are varying in every wavefront". For instance, if required workgroup dimensions are known, we could consider only the workitem.id intrinsics for dimensions that vary within a wavefront where the current PR checks for sources of divergence.

This means that execz branches that would be taken may be removed; the actual removal only happens if it doesn't affect correctness. 

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