Change the behaviour of the cost-centre stack profiler for recursive
sccs.  Before, we used to remove the old copy of the CC from the stack
and push the new one on.  Now, we record back-edges in the graph when
a recursive scc is detected.

This should give saner-looking cost-centre stacks for heavily
recursive code.  Indeed, it reduces the number of stacks in the
profile for some examples considerably.