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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> LLVM intrinsics currently use a different mechanism, calling into a target hook to determine their uniformity characteristics. But maybe that could be changed so that you can declare an intrinsic as IntrAlwaysUniform and have that automatically set the alwaysuniform attribute added in this PR.

This would eliminate a hand maintained switch I suppose 


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