[llvm] AtomicExpand: Allow incrementally legalizing atomicrmw (PR #103371)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Aug 30, 11:09 AM EDT**: @arsenm started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/103371).


https://github.com/llvm/llvm-project/pull/103371