[llvm] [AMDGPU] Mark `SI_WATERFALL_LOOP` as divergent in `hasDivergentBranch` (PR #218969)

[Jay Foad via llvm-commits]

https://github.com/jayfoad approved this pull request.

Makes sense to me. This is temporal divergence (%3 is uniform inside the loop and the exit condition is divergent) and the mechanism we have for preventing machine-sink from doing this is isSafeToSink caling hasDivergentBranch.

https://github.com/llvm/llvm-project/pull/218969