Eliminate gcc7 compiler warnings

we will get warning:```parameter ‘op’ set but not used``` when template function with empty template parameter list.

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D122527

mlir/include/mlir/IR/OpDefinition.h

@@ -1617,6 +1617,7 @@ static LogicalResult verifyTraits(Operation *op) {
 template <typename... Ts>
 static LogicalResult verifyRegionTraitsImpl(Operation *op,
                                             std::tuple<Ts...> *) {
+  (void)op;
   LogicalResult result = success();
   (void)std::initializer_list<int>{
       (result = succeeded(result) ? Ts::verifyRegionTrait(op) : failure(),