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

[Igor Wodiany via llvm-commits]

IgWod wrote:

I've rebased the change on the pre-commited test, so the impact is clearly visible. @shiltian @krzysz00 Are you happy with the change or do you have any more comments/questions/concerns?

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