[llvm] [InstCombine] Optimize AMDGPU ballot + assume uniformity patterns (PR #160670)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm requested changes to this pull request.

Previous questions unanswered. I don't understand why this would be performed in the context of an assume, and not on the free-form pattern. Also, I would expect the assume to inform uniformity analysis, not just be dropped here 

https://github.com/llvm/llvm-project/pull/160670