[llvm] [Uniformity] Avoid marking all PHIs as divergent in join blocks (PR #157808)

[Carl Ritson via llvm-commits]

perlfu wrote:

> Is this patch still useful?

I still think we want to do this.

I have not been pushing on it at the moment as the uniformity analysis deficiencies I am seeing are more extensive than this patch and require further analysis.  Essentially there are certain control structures which are obviously uniform but labelled as divergent.

https://github.com/llvm/llvm-project/pull/157808