// larger combined cluster is represented by `cluster_from`, but can have
  // `cycles_graph_`'s ID of either `cluster_from` or `cluster_to` depending on
  // which way will require less operations.
  bool MergeClusters(Cluster* cluster_from, Cluster* cluster_to) {
    int from = cluster_from->cycles_graph_node_id();
    int to = cluster_to->cycles_graph_node_id();

    auto optional_merged_node = cycles_graph_.ContractEdge(from, to);