Dept of second thoughts: don't try to push LIMIT below a SRF.

If we have Limit->Result->Sort, the Result might be projecting a tlist
that contains a set-returning function.  If so, it's possible for the
SRF to sometimes return zero rows, which means we could need to fetch
more than N rows from the Sort in order to satisfy LIMIT N.
So top-N sorting cannot be used in this scenario.