// Look for either _XlaScope or _XlaInternalScope on both nodes to guide
  // clustering.  If both nodes have a scope and the scopes do not match, do
  // not cluster along this edge.  If even one of the nodes lacks a scope
  // attribute, then it is treated as a "bridge" and a cluster may be created
  // along it.
  //
  // The difference between _XlaScope and _XlaInternalScope is that _XlaScope is