[regex] Speed up somewhat the cases in bug-regex11.c

[Paolo Bonzini]

The real solution to these bugs would be to save the set of
epsilon-reachable OP_OPEN_SUBEXP and OP_CLOSE_SUBEXP, and store them into
the backreference cache.

But in the meanwhile, this patch can already speed up the problematic
function by a large amount.  search_cur_bkref_entry disappears from the
profile, and on a reduced testcase (intermediate between the succeeding and
failing testcases) check_dst_limits_calc_pos is cut to about half the time
it used to take.

This is done by hoisting most checks in check_dst_limits_calc_pos out of the
recursive calls, and by optimizing the loops on the backreference cache
entries.  These loops have two exit conditions, end-of-backref-cache and
got-past-this-str_idx: by flagging specially the last backreference cache
entry for each str_idx, we can test the first condition on the first
iteration, and the special flag on every subsequent iteration.

Can you please apply all these patches except the one Bruno remarked on?  I
am working out with him a better solution for that.

Paolo

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