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

[Fabian Ritter via llvm-commits]

ritter-x2a wrote:

@jayfoad @arsenm I made the heuristic more selective in 8cf0cbf8fb44c9630cb48f51f3b2e22b68598272, it can now distinguish the example cases I mentioned above.

I haven't found a good alternative for using the branch weights yet since AFAIK normal IR metadata does not find its way to the backend. We could add a boolean `isLikelyVarying` parameter to the `amdgcn_if` and `amdgcn_else` intrinsics (I think that, in that case, moving the functionality from this pass into SIAnnotateControlFlow would make more sense).

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