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);
    if (!optional_merged_node.has_value()) {
      VLOG(3) << "Could not contract " << cluster_from->DebugString(*graph_)
              << " -> " << cluster_to->DebugString(*graph_)