message: speed up clear_cfg
  Summary:
  Before: clear cfg is ~22% of total (inclusive) cycles
  {F123878493}
  After: clear_cfg is ~2% of total (inclusive) cycles. It's the small box on the right side that's barely visible :D
  {F123878494}
diff: @@ -577,19 +577,21 @@ void IRCode::build_cfg(bool editable) {
  }
  void IRCode::clear_cfg() {
  - if (m_cfg && m_cfg->editable()) {
  + if (!m_cfg) {
  + return;
  + }
  +
  + if (m_cfg->editable()) {
  m_ir_list = m_cfg->linearize();
  }
  m_cfg.reset();
  - std::vector<IRList::iterator> fallthroughs;
  - for (auto it = m_ir_list->begin(); it != m_ir_list->end(); ++it) {
  + for (auto it = m_ir_list->begin(); it != m_ir_list->end();) {
  if (it->type == MFLOW_FALLTHROUGH) {
  - fallthroughs.emplace_back(it);
  - }
  + it = m_ir_list->erase_and_dispose(it);
  + } else {
  + ++it;
  }
  - for (auto it : fallthroughs) {
  - m_ir_list->erase_and_dispose(it);
  }
  }
  