/// form register groups. A node is represented by an index into
/// the vector. A node can "point to" itself to indicate that it
/// is the parent of a group, or point to another node to indicate
/// that it is a member of the same group as that node.
std::vector<unsigned> GroupNodes;
/// form register groups. A node is represented by an index into
/// the vector. A node can "point to" itself to indicate that it
/// is the parent of a group, or point to another node to indicate
/// that it is a member of the same group as that node.
std::vector<unsigned> GroupNodes;