further simplify (??{}) return code

further simplify (??{}) return code

This does two things. First, it conflates the RV and non-RV cases; i.e.

    if (ROK) {
        sv = RV(sv)
        if (REGEXP)  rx = sv;
        else if (PERL_MAGIC_qr) rx = mg_obj;
    }
    else {
        if (REGEXP)  rx = sv;
        else if (PERL_MAGIC_qr) rx = mg_obj;
    }

becomes

    if (ROK)
        sv = RV(sv)
    if (REGEXP)  rx = sv;
    else if (PERL_MAGIC_qr) rx = mg_obj;

Secondly, elmininate the intermediate rx var; it's only point in life is
to be assigned to re_sv, so just use re_sv throughout.