Replace dyn_cast<>() with isa<>() when the result isn't used (NFC)

Fixed warning reported by some GCC version.

mlir/lib/TableGen/Operator.cpp

@@ -315,7 +315,7 @@ void tblgen::Operator::populateTypeInferenceInfo(
         continue;
       }
 
-      if (auto *attr = getArg(*mi).dyn_cast<NamedAttribute *>()) {
+      if (getArg(*mi).isa<NamedAttribute *>()) {
         // TODO: Handle attributes.
         continue;
       } else {