[DAG] Remove non-canonical AVG case.

This removes a condition in the detection of AVG nodes, where we needn't be
checking the LHS of an add node as any const will be canonicalized to the RHS.

Author: davemgreen

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -947,13 +947,6 @@ static SDValue combineShiftToAVG(SDValue Op, SelectionDAG &DAG,
   SDValue Add2;
   auto MatchOperands = [&](SDValue Op1, SDValue Op2, SDValue Op3, SDValue A) {
     ConstantSDNode *ConstOp;
-    if ((ConstOp = isConstOrConstSplat(Op1, DemandedElts)) &&
-        ConstOp->isOne()) {
-      ExtOpA = Op2;
-      ExtOpB = Op3;
-      Add2 = A;
-      return true;
-    }
     if ((ConstOp = isConstOrConstSplat(Op2, DemandedElts)) &&
         ConstOp->isOne()) {
       ExtOpA = Op1;