[llvm] AMDGPU: Extract lambda used in foldImmediate into a helper function (PR #127484)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Feb 18, 5:11 AM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/127484).


https://github.com/llvm/llvm-project/pull/127484