[llvm] [IR] Add `alwaysuniform` function attribute (PR #214644)

[Reem Elkhouly via llvm-commits]

amd-relkhoul wrote:

> Oh I see. I think the intended use case is for functions that are not LLVM intrinsics. By "a downstream ballot intrinsic" I think Reem meant a function that has some special meaning to the frontend, not an intrinsic in the LLVM sense.

Yes, the use case is a frontend (intrinsic-like) function that gets lowered by frontend pass. It is not LLVM intrinsic.
I'll edit the description to remove the confusing term.

https://github.com/llvm/llvm-project/pull/214644