[llvm] AMDGPU/UniformityAnalysis: fix G_ZEXTLOAD and G_SEXTLOAD (PR #157845)

[Petar Avramovic via llvm-commits]

petar-avramovic wrote:

### Merge activity

* **Sep 10, 3:55 PM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/157845).


https://github.com/llvm/llvm-project/pull/157845