[llvm] LowerAtomic: Use explicit alignment in lowerAtomicCmpXchgInst (PR #103767)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Aug 14, 9:00 AM EDT**: @arsenm started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/103767).


https://github.com/llvm/llvm-project/pull/103767