message: fix crash with phis with multiple edges from same BB
diff: @@ -163,10 +163,11 @@ const BasicBlock& Function::bbOf(const Instr &i) const {
  BasicBlock& Function::insertBBBefore(string_view name, const BasicBlock &bb) {
  auto p = BBs.try_emplace(string(name), name);
  - assert(p.second);
  + if (p.second) {
  auto I = find(BB_order.begin(), BB_order.end(), &bb);
  assert(I != BB_order.end());
  BB_order.insert(I, &p.first->second);
  + }
  return p.first->second;
  }
  