[llvm] AMDGPU: Add uniformity analysis test for G_ZEXTLOAD and G_SEXTLOAD (PR #157844)

[Petar Avramovic via llvm-commits]

petar-avramovic wrote:

### Merge activity

* **Sep 10, 1:05 PM UTC**: [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/157844) rebased this pull request as part of a merge.


https://github.com/llvm/llvm-project/pull/157844