[llvm] AMDGPU: Add baseline test for add64 with constant test (PR #122048)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Jan 8, 10:24 AM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/122048).


https://github.com/llvm/llvm-project/pull/122048