move intuit call from pp_match() into regexec()

move intuit call from pp_match() into regexec()

Currently the main part of pp_match() looks like:

    if (can_use_intuit) {
        if (!intuit_start())
            goto nope;
        if (can_match_based_only_on_intuit_result) {
            ... set up $&, $-[0] etc ...
            goto gotcha;
        }
    }
    if (!regexec(..., REXEC_CHECKED|r_flags))
        goto nope;

  gotcha:
    ...

This rather breaks the regex API encapulation. The caller of the regex
engine shouldn't have to worry about whether to call intuit() or
regexec(), and to know to set $& in the intuit-only case.

So, move all the intuit-calling and $& setting into regexec itself.
This is cleaner, and will also shortly allow us to enable intuit-only
matches in pp_subst() too. After this change, the code above looks like
(in its entirety):

    if (!regexec(..., r_flags))
        goto nope;

    ...

There, isn't that nicer?