[SelectionDAGBuilder] Use APInt::isZero instead of comparing APInt::getZExtValue to 0 in a place where we can't be sure contents of the APInt fit in a uint64_t.

This is used on an extract vector element index which is most cases is going to be an i32 or i64 and the element will be a valid element number. But it is possible to construct IR with a larger type and large out of range value.

llvm-svn: 337652

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -2757,7 +2757,7 @@ static bool isVectorReductionOp(const User *I) {
           return false;
 
         const ConstantInt *Val = dyn_cast<ConstantInt>(U->getOperand(1));
-        if (!Val || Val->getZExtValue() != 0)
+        if (!Val || !Val->isZero())
           return false;
 
         ReduxExtracted = true;