re_intuit_start(): update rx_origin after check

re_intuit_start(): update rx_origin after check

Previously the code for updating rx_origin after a 'check' match or an
'other' match looked a bit like this:

    s = fbm_instr(check);

    if (other exists) {
        if (other is anchored) {
            rx_origin = HOP3c(s, -prog->check_offset_max);
            ....
        }
        else {
            rx_origin = HOP3c(s, -prog->check_offset_min);
            ....
        }
    }
    else
        rx_origin = HOP3c(s, -prog->check_offset_max);

This commit changes it to

    s = fbm_instr(check);
    rx_origin = HOP3c(s, -prog->check_offset_max);

    if (other exists) {
        if (other is anchored) {
            ....
        }
        else {
            ....
        }
    }

Of course in each case the 'HOP3' code was slightly different, but they
all happened to be equivalent, especially as for an anchored string,
check_offset_min == check_offset_max.

The only complication was a goto do_other_anchored, but it turns
out that setting rx_origin in that case was easy.