[llvm] [UniformityAnalysis] Add test for missed cycle exit PHI divergence (PR #216100)

[Sameer Sahasrabuddhe via llvm-commits]

https://github.com/ssahasra commented:

This _should_ be submitted as a precursor to #216296. But I am not sure if negative tests should be marked XFAIL instead of pretending to check the wrong output as correct.

https://github.com/llvm/llvm-project/pull/216100