[X86] Don't dereference a dyn_cast<> - use a cast<> instead. NFCI.

dyn_cast<> can return null if the cast fails, by using cast<> we assert that the cast is correct helping to avoid a potential null dereference.

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -44378,8 +44378,7 @@ static SDValue combineAndLoadToBZHI(SDNode *Node, SelectionDAG &DAG,
           uint64_t ArrayElementCount = Init->getType()->getArrayNumElements();
           bool ConstantsMatch = true;
           for (uint64_t j = 0; j < ArrayElementCount; j++) {
-            ConstantInt *Elem =
-                dyn_cast<ConstantInt>(Init->getAggregateElement(j));
+            auto *Elem = cast<ConstantInt>(Init->getAggregateElement(j));
             if (Elem->getZExtValue() != (((uint64_t)1 << j) - 1)) {
               ConstantsMatch = false;
               break;