message: Fix iterator invalidation when pushing a self-reference on a SmallVector
  This crashes with
diff: @@ -677,7 +677,7 @@ ParseResult ParseMergeOp(OpAsmParser &parser, OperationState &result) {
  } else {
  // In case of the short form, use the parsed type for both the operands and
  // the remaining operands are expected to be control inputs.
  - types.push_back(types.front());
  + types.push_back(Type(types.front()));
  Type control_type = ControlType::get(parser.getBuilder().getContext());
  types.append(op_infos.size() - 2, control_type);
  