[VectorUtils] Use PatternMatch in findScalarElement (NFC) (#168389)

Author: artagnon

llvm/lib/Analysis/VectorUtils.cpp

@@ -317,9 +317,9 @@ Value *llvm::findScalarElement(Value *V, unsigned EltNo) {
 
   if (InsertElementInst *III = dyn_cast<InsertElementInst>(V)) {
     // If this is an insert to a variable element, we don't know what it is.
-    if (!isa<ConstantInt>(III->getOperand(2)))
+    uint64_t IIElt;
+    if (!match(III->getOperand(2), m_ConstantInt(IIElt)))
       return nullptr;
-    unsigned IIElt = cast<ConstantInt>(III->getOperand(2))->getZExtValue();
 
     // If this is an insert to the element we are looking for, return the
     // inserted value.