rangesel() bugs on basic comparison operators with an empty range

I found a few bugs on the selectivity estimation function for
the range types, rangesel() for empty ranges.

# create table a (b int4range);
CREATE TABLE

# select * from a where b < 'empty';
ERROR:  unexpected operator 3884

# select * from a where b >= 'empty';
ERROR:  unexpected operator 3886

Patch attached.