[DAG] Remove (dead) legalization for atomic LoadSDNode and StoreSDNode

This should have been part of 943f3e5 which removed the never completed
migration code which added these.  I left them out because I thought there
was more generic SDAG code to cleanup, but I'd forgotten that SystemZ
relied on custom legalizing ATOMIC_LOAD to (atomic) LoadSDNode.  As a
result, we still need the various legality checks on combines and the
common infrastructure to suport them.

Author: preames

llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp

@@ -3830,20 +3830,7 @@ void DAGTypeLegalizer::ExpandIntRes_XROUND_XRINT(SDNode *N, SDValue &Lo,
 
 void DAGTypeLegalizer::ExpandIntRes_LOAD(LoadSDNode *N,
                                          SDValue &Lo, SDValue &Hi) {
-  if (N->isAtomic()) {
-    // It's typical to have larger CAS than atomic load instructions.
-    SDLoc dl(N);
-    EVT VT = N->getMemoryVT();
-    SDVTList VTs = DAG.getVTList(VT, MVT::i1, MVT::Other);
-    SDValue Zero = DAG.getConstant(0, dl, VT);
-    SDValue Swap = DAG.getAtomicCmpSwap(
-        ISD::ATOMIC_CMP_SWAP_WITH_SUCCESS, dl,
-        VT, VTs, N->getOperand(0),
-        N->getOperand(1), Zero, Zero, N->getMemOperand());
-    ReplaceValueWith(SDValue(N, 0), Swap.getValue(0));
-    ReplaceValueWith(SDValue(N, 1), Swap.getValue(2));
-    return;
-  }
+  assert(!N->isAtomic() && "Should have been a ATOMIC_LOAD?");
 
   if (ISD::isNormalLoad(N)) {
     ExpandRes_NormalLoad(N, Lo, Hi);
@@ -5398,16 +5385,8 @@ SDValue DAGTypeLegalizer::ExpandIntOp_XINT_TO_FP(SDNode *N) {
 }
 
 SDValue DAGTypeLegalizer::ExpandIntOp_STORE(StoreSDNode *N, unsigned OpNo) {
-  if (N->isAtomic()) {
-    // It's typical to have larger CAS than atomic store instructions.
-    SDLoc dl(N);
-    SDValue Swap = DAG.getAtomic(ISD::ATOMIC_SWAP, dl,
-                                 N->getMemoryVT(),
-                                 N->getOperand(0), N->getOperand(2),
-                                 N->getOperand(1),
-                                 N->getMemOperand());
-    return Swap.getValue(1);
-  }
+  assert(!N->isAtomic() && "Should have been a ATOMIC_STORE?");
+
   if (ISD::isNormalStore(N))
     return ExpandOp_NormalStore(N, OpNo);