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

[Sameer Sahasrabuddhe via llvm-commits]

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+; RUN: opt %s -mtriple amdgcn-- -passes='print<uniformity>' -disable-output 2>&1 | FileCheck %s
+
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ssahasra wrote:

Would be useful to have one or two lines explaining what's the distinguishing pattern in each function compared to other functions in the file.

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