message: Implement special case of br in terms of return
diff: @@ -334,6 +334,9 @@ execution_result execute(Instance& instance, FuncIdx func_idx, std::vector<uint6
  if (instruction == Instr::br_if && static_cast<uint32_t>(stack.pop()) == 0)
  break;
  + if (label_idx == labels.size())
  + goto case_return;
  +
  assert(labels.size() > label_idx);
  labels.drop(label_idx); // Drop skipped labels (does nothing for labelidx == 0).
  const auto label = labels.pop();
  @@ -389,6 +392,7 @@ execution_result execute(Instance& instance, FuncIdx func_idx, std::vector<uint6
  break;
  }
  case Instr::return_:
  + case_return:
  {
  // TODO: Not needed, but satisfies the assert in the end of the main loop.
  labels.clear();
  