Quick hack to allow the outer query's tuple_fraction to be passed down

Quick hack to allow the outer query's tuple_fraction to be passed down
to a subquery if the outer query is simple enough that the LIMIT can
be reflected directly to the subquery.  This didn't use to be very
interesting, because a subquery that couldn't have been flattened into
the upper query was usually not going to be very responsive to
tuple_fraction anyway.  But with new code that allows UNION ALL subqueries
to pay attention to tuple_fraction, this is useful to do.  In particular
this lets the optimization occur when the UNION ALL is directly inside
a view.