Here is current comment:
  // Set/get control node out. Set lower bit to distinguish from IdealLoopTree
  // Returns true if "n" is a data node, false if it's a control node.

I think it is opposite! has_ctrl(n) returns true if it's a control node (the lowest bit is set). it returns false when it is a IdealLoopTree* data.

We can deduce this from real code. eg.
1. getLoop() asserts that has_ctrl(n) == false
  IdealLoopTree *get_loop( Node *n ) const {
    // Dead nodes have no loop, so return the top level loop instead
    if (!has_node(n)) return _ltree_root;
    assert(!has_ctrl(n), "");
    return (IdealLoopTree*)_nodes[n->_idx];
  }

2. when we set ctrl, either n is dead or has_ctrl(n) is true.
  void set_ctrl( Node *n, Node *ctrl ) {
    assert( !has_node(n) || has_ctrl(n), "" );
    assert( ctrl->in(0), "cannot set dead control node" );
    assert( ctrl == find_non_split_ctrl(ctrl), "must set legal crtl" );
    _nodes.map( n->_idx, (Node*)((intptr_t)ctrl + 1) );
  }


update:
I misunderstood the comment. if we define control node is "a CFG node or pinned node", then yes, return false if it's a control node.