See D66309 for context.
This is the first sweep of generic code to add isAtomic bailouts where appropriate. The intention here is to have the switch from AtomicSDNode to LoadSDNode/StoreSDNode be close to NFC; that is, I'm not looking to allow additional optimizations at this time. That will come later.
(Note: This is currently based on ToT without D66309. That's why there are no test diffs with the experimental flag visible. It could be added either before or after that change.)