[llvm] AMDGPU: Add more tests for fmed3 instcombine folds (PR #139529)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **May 12, 2:11 PM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/139529).


https://github.com/llvm/llvm-project/pull/139529