bug#65726: 29.1.50; Crash in regexp engine

[Stefan Monnier]

> I think you are right, but wouldn't mind seeing it confirmed
> empirically. Say, by cross-checking using an an alternative (slower)
> implementation that directly checks whether a node has been visited before.

I'm reworking my code so it takes 2 arguments: the loop-entry and the
loop-exit.  Any jump outside of those bounds should never happen so we
can assert it to check that our assumptions are right (and we can
return false when we don't compile assertions, so it's never over-optimistic).


        Stefan