[llvm] AMDGPU: Add cost model tests for minimumnum/maximumnum (PR #141904)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Jun 17, 10:54 PM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/141904).


https://github.com/llvm/llvm-project/pull/141904