Rename find_em_expr_usable_for_sorting_rel.

 Rename find_em_expr_usable_for_sorting_rel.
 
 I didn't particularly like this function name, as it fails to
 express what's going on.  Also, returning the sort expression
 alone isn't too helpful --- typically, a caller would also
 need some other fields of the EquivalenceMember.  But the
 sole caller really only needs a bool result, so let's make
 it "bool relation_can_be_sorted_early()".
 
 Discussion: https://postgr.es/m/91f3ec99-85a4-fa55-ea74-33f85a5c651f@swarm64.com