[llvm] AtomicExpand: Refactor atomic instruction handling (PR #102914)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Aug 13, 11:50 AM EDT**: @arsenm started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/102914).


https://github.com/llvm/llvm-project/pull/102914