Use a hopefully-more-reliable method of detecting default selectivity

estimates when combining the estimates for a range query.  As pointed out
by Miquel van Smoorenburg, the existing check for an impossible combined
result would quite possibly fail to detect one default and one non-default
input.  It seems better to use the default range query estimate in such
cases.  To do so, add a check for an estimate of exactly DEFAULT_INEQ_SEL.
This is a bit ugly because it introduces additional coupling between
clauselist_selectivity and scalarltsel/scalargtsel, but it's not like
there wasn't plenty already...