[X86] Use isNullConstant (NFC)

Author: kazutakahirata

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -55243,11 +55243,10 @@ static SDValue combineScalarToVector(SDNode *N, SelectionDAG &DAG) {
   // Combine scalar_to_vector of an extract_vector_elt into an extract_subvec.
   if (VT == MVT::v1i1 && Src.getOpcode() == ISD::EXTRACT_VECTOR_ELT &&
       Src.hasOneUse() && Src.getOperand(0).getValueType().isVector() &&
-      Src.getOperand(0).getValueType().getVectorElementType() == MVT::i1)
-    if (auto *C = dyn_cast<ConstantSDNode>(Src.getOperand(1)))
-      if (C->isZero())
-        return DAG.getNode(ISD::EXTRACT_SUBVECTOR, DL, VT, Src.getOperand(0),
-                           Src.getOperand(1));
+      Src.getOperand(0).getValueType().getVectorElementType() == MVT::i1 &&
+      isNullConstant(Src.getOperand(1)))
+    return DAG.getNode(ISD::EXTRACT_SUBVECTOR, DL, VT, Src.getOperand(0),
+                       Src.getOperand(1));
 
   // Reduce v2i64 to v4i32 if we don't need the upper bits or are known zero.
   // TODO: Move to DAGCombine/SimplifyDemandedBits?