[Patch 2/3] Make regex's infinite loop detection accurate

[Tim Shen]

regex_match("", regex("(?:)*") will cause segfault because the
infinite loop detection (writeen by me) is stupid. :O

Instead of checking begins and ends, now the executor tracks repeating
count for every loop node (typically generated from the Kleene star
*).

It slows down the executor a bit (from 1.8s to 2.0s in my machine),
but it's about the correctness.

The whole patch series are booted and tested, but this one is not
separately tested.

Thanks!


-- 
Regards,
Tim Shen