[llvm] AMDGPU/GlobalISel: add mir test for sgpr s16 unmerge (PR #179440)

[Petar Avramovic via llvm-commits]

petar-avramovic wrote:

### Merge activity

* **Feb 3, 3:26 PM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.com/github/pr/llvm/llvm-project/179440).


https://github.com/llvm/llvm-project/pull/179440