[llvm] [WIP][Assume] Generic assume-based uniformity optimization (PR #160670)

[Sameer Sahasrabuddhe via llvm-commits]

https://github.com/ssahasra commented:

Still need to answer whether this is the right place for doing this transform. Also, another feature request for future work:

> If `ballot == -1` appears inside a conditional branch, then UA should assume that the branch condition is uniform.

This is a whole new enhancement, and not in scope for the current patch.

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