The fix for 6614330 introduced a performance problem in Node::dump() when
whole graph is dumping (it takes forever to dump it).
I replaced depth-first iteration with breadth-first.
The code is very simple and fast. But I had to remove
Ross's search for loops (which don't have phis and regions).
I also fxied the old bug: Node_Stack.size() incorrectly
returns big number instead of 0 when it is empty.