[llvm] AMDGPU: Add baseline tests for bitcast + readlane intrinsics (PR #128493)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Feb 26, 2:22 AM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/128493).


https://github.com/llvm/llvm-project/pull/128493