[llvm] MachineUniformityAnalysis: Improve isConstantOrUndefValuePhi (PR #112866)

[Petar Avramovic via llvm-commits]

petar-avramovic wrote:

### Merge activity

* **Jan 24, 6:41 AM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/112866).


https://github.com/llvm/llvm-project/pull/112866