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

[Krzysztof Drewniak via llvm-commits]

krzysz00 wrote:

I'm guessing lanes that were already inactive when you enter the loop aren't going to take the loop?

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