message: Do not remove the entry block when simplifying
  Summary:
  A CFG needs an entry block. Do not remove it even if it is empty.
  Fixes a use-after-free.
diff: @@ -965,6 +965,12 @@ void ControlFlowGraph::remove_empty_blocks() {
  m_entry_block = succ;
  }
  }
  + if (b == m_entry_block) {
  + // Don't delete the entry block. If it was empty and had a successor,
  + // we'd have replaced it just above.
  + ++it;
  + continue;
  + }
  for (const auto& mie : *b) {
  if (mie.type == MFLOW_POSITION) {
  