re_intuit_start(): remove try_at_* labels

re_intuit_start(): remove try_at_* labels

Now that both "other" blocks have been merged into one block, there's
only one occurrence of the following rather than two:

    if (rx_origin == strpos)
        goto try_at_start;
    goto try_at_offset;

which allows us to eliminate these two gotos and just fall through into
the 'if (rx_origin == strpos)' just before the two code blocks marked by
those two labels.

Also intro introduce another label, 'postprocess_substr_matches',
which is needed by the stclass code now that those other two labels have
gone.