re_intuit_start(): check for IMPLICIT in abs anch

re_intuit_start(): check for IMPLICIT in abs anch

The block of code that deals with absolute anchors looks like:

    if (...) {
        if (.... && !PREGf_IMPLICIT) ...;
        if (FOO) {
            assert(!PREGf_IMPLICIT);
            ....
        }
    }

where the constraints of FOO imply PREGf_IMPLICIT, as shown by the
assertion. Simplify this to:

    if (... && !PREGf_IMPLICIT) {
        if (....) ...;
        if (FOO) {
            ....
        }
    }