SelectionDAG: Swap operands of atomic_store

Irritatingly, atomic_store had operands in the opposite order from
regular store. This made it difficult to share patterns between
regular and atomic stores.

There was a previous incomplete attempt to move atomic_store into the
regular StoreSDNode which would be better.

I think it was a mistake for all atomicrmw to swap the operand order,
so maybe it's better to take this one step further.

https://reviews.llvm.org/D123143