message: simplify insert-order merging
  Summary:
  A couple of properties are true by construction, and we don't need to check for them. In other words, this deletes some dead code, and instead checks new assertions.
  This is a behavior-preserving change.
diff: @@ -520,14 +520,17 @@ void AliasedRegisters::handle_insert_order_at_merge(
  const Value& val_a = this->m_graph[a];
  const Value& val_b = this->m_graph[b];
  - bool this_has = this->vertices_are_aliases(a, b);
  + always_assert_log(this->vertices_are_aliases(a, b),
  + "by construction, vertices in a merged group are aliases "
  + "in this graph");
  boost::optional<vertex_t> other_a = other.find(val_a);
  boost::optional<vertex_t> other_b =
  (other_a == boost::none ? boost::none
  : other.find_in_tree(val_b, *other_a));
  - bool other_has = other_b != boost::none;
  + always_assert_log(other_b != boost::none,
  + "by construction, vertices in a merged group are aliases "
  + "in other graph");
  - if (this_has && other_has) {
  // Intersection case should always come here
  bool this_less_than =
  this->m_insert_order.at(a) < this->m_insert_order.at(b);
  @@ -543,15 +546,6 @@ void AliasedRegisters::handle_insert_order_at_merge(
  // The graphs do not agree. Choose a deterministic order
  return val_a.reg() < val_b.reg();
  }
  - } else if (this_has) {
  - return this->m_insert_order.at(a) < this->m_insert_order.at(b);
  - } else if (other_has) {
  - return other.m_insert_order.at(*other_a) <
  - other.m_insert_order.at(*other_b);
  - } else {
  - // Neither graph has this relationship. Choose a deterministic order
  - return val_a.reg() < val_b.reg();
  - }
  });
  }
  