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

[Jay Foad via llvm-commits]

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@@ -189,3 +189,54 @@ body: |
     FLAT_STORE_DWORD %3, %11, 0, 0, implicit $exec, implicit $flat_scr :: (store (s32))
     SI_RETURN
 ...
+
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jayfoad wrote:

Can you precommit this (or use stacked PRs or at least two commits in one PR) so we can see the effect of your patch?

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