[SelectionDAG] Refactor lowering of atomic memory intrinsics.

Summary:
This just refactors the lowering of the atomic memory intrinsics to more
closely match the code patterns used in the lowering of the non-atomic
memory intrinsics. Specifically, we encapsulate the lowering in
SelectionDAG::getAtomicMem*() functions rather than embedding
the code directly in the SelectionDAGBuilder code.

llvm-svn: 330603