Relationship functions not using an index in some cases

Relationship functions not using an index in some cases
Set cost back to 1 for functions that return a geometry
A higher cost has the side-effect of making the planner
think when used in conjunction with relate constructs like ST_Intersects
it's cheaper to compute the geometry once rather
than compute twice and break into an index and non-index call.
References #3675 for PostGIS 2.4

git-svn-id: http://svn.osgeo.org/postgis/trunk@15281 b70326c6-7e19-0410-871a-916f4a2858ee