message: Cleanup remaining refs
  Summary:
  This strenghtens internal invariants and frees up memory faster.
  This is a behavior-preserving change.
diff: @@ -394,7 +394,9 @@ size_t CrossDexRefMinimizer::erase(DexClass* cls, bool emitted, bool reset) {
  for (const auto& p : refs) {
  auto ref = p.first;
  uint32_t weight = p.second;
  - auto& classes = m_ref_classes.at(ref);
  + auto classes_it = m_ref_classes.find(ref);
  + always_assert(classes_it != m_ref_classes.end());
  + auto& classes = classes_it->second;
  size_t frequency = classes.size();
  always_assert(frequency > 0);
  const auto erased = classes.erase(cls);
  @@ -406,7 +408,9 @@ size_t CrossDexRefMinimizer::erase(DexClass* cls, bool emitted, bool reset) {
  }
  }
  --frequency;
  - if (frequency > 0 && frequency <= INFREQUENT_REFS_COUNT) {
  + if (frequency == 0) {
  + m_ref_classes.erase(classes_it);
  + } else if (frequency <= INFREQUENT_REFS_COUNT) {
  for (DexClass* affected_class : classes) {
  affected_classes[affected_class]
  .infrequent_refs_weight[frequency - 1] += weight;
  