[SDAG] Update generic code to conservatively check for isAtomic in addition to isVolatile

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.  See D66309 for context.

Differential Revision: https://reviews.llvm.org/D66318

llvm-svn: 371786