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

[Fabian Ritter via llvm-commits]

ritter-x2a wrote:

> How can branch weights tell you whether the branch is likely uniform or not? I guess if it's likely uniform-and-true or uniform-and-false then they can tell you that.

This patch doesn't use branch weights to find something out, it sets them to signal (to the later SIPreEmitPeephole pass) that it's likely that 'then' and 'else' case are executed, i.e., that it should remove the execz branches if possible.

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