[llvm] AMDGPU: Add baseline test for copysign combine (PR #97150)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Jul 1, 6:08 AM EDT**: @arsenm started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/97150).


https://github.com/llvm/llvm-project/pull/97150