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

[Fabian Ritter via llvm-commits]

ritter-x2a wrote:

> Not so sure about this. The compiler has no information about the contents of `lookup[tid]`, so no idea how likely it is that the condition will be false for every lane, correct?

Agreed. That's why I suggest keeping the handling for this case as it is in trunk and not eliminate the execz branch for it.

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