[DAGCombine] Use FoldConstantArithmetic() to perform ((c1-A)+c2) -> (c1+c2)-A fold

[lebedev.ri]

Summary

No tests change, and i'm not sure how to test this, but it's better safe than sorry.

[craig.topper]

I'm not sure what this FIXME was trying to say. getNode should detect the constants and call FoldConstantArithmetic anyway. @spatel?

[craig.topper]

LGTM. I dont' think this causes a functional change. At best its a very minor compile time improvement.

llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

SDValue DAGCombiner::visitADDLike(SDNode *N) {
   // fold (add x, 0) -> x
   if (isNullConstant(N1))
     return N0;
 
   if (isConstantOrConstantVector(N1, /* NoOpaque */ true)) {
     // fold ((c1-A)+c2) -> (c1+c2)-A
     if (N0.getOpcode() == ISD::SUB &&
         isConstantOrConstantVector(N0.getOperand(0), /* NoOpaque */ true)) {
-      // FIXME: Adding 2 constants should be handled by FoldConstantArithmetic.
-      return DAG.getNode(ISD::SUB, DL, VT,
-                         DAG.getNode(ISD::ADD, DL, VT, N1, N0.getOperand(0)),
-                         N0.getOperand(1));
+      SDValue Add = DAG.FoldConstantArithmetic(ISD::ADD, DL, VT, N1.getNode(),
+                                               N0.getOperand(0).getNode());
+      assert(Add && "Constant folding failed");
+      return DAG.getNode(ISD::SUB, DL, VT, Add, N0.getOperand(1));
     }
 
     // add (sext i1 X), 1 -> zext (not i1 X)
     // We don't transform this pattern:
     //   add (zext i1 X), -1 -> sext (not i1 X)
     // because most (?) targets generate better code for the zext form.
     if (N0.getOpcode() == ISD::SIGN_EXTEND && N0.hasOneUse() &&
         isOneOrOneSplat(N1)) {