[SDAG] Add a isSimple cover functon to MemSDNode, just as we have in IR/MI [NFC]

Uses are in reviews D66322 and D66318.  Submitted separately to control rebuild times.

llvm-svn: 371445

llvm/include/llvm/CodeGen/SelectionDAGNodes.h

@@ -1361,6 +1361,9 @@ public:
   /// aliasing rules.
   bool isUnordered() const { return MMO->isUnordered(); }
 
+  /// Returns true if the memory operation is neither atomic or volatile.
+  bool isSimple() const { return !isAtomic() && !isVolatile(); }
+
   /// Return the type of the in-memory value.
   EVT getMemoryVT() const { return MemoryVT; }