Background: At the moment, we record the AtomicOrdering of an access in the MMO, but also mark any atomic access as volatile in SelectionDAG. I'm working towards separating that. See https://reviews.llvm.org/D57601 for context.
Update all usages of isVolatile in lib/CodeGen to preserve behaviour once atomic MMOs stop being also volatile. This is NFC in it's current form, but is essential for correctness once we make that final change.
It useful to keep in mind that AtomicSDNode is not a parent of LoadSDNode, StoreSDNode, or LSBaseSDNode. As a result, any call to isVolatile on one of those static types doesn't need a companion isAtomic check.
I'm deliberately being conservative about handling. I want the change to stop adding volatile to be NFC itself, and then will work through places where we can be less conservative for atomics one by one in separate changes w/tests.