Add get_opfamily_name() function
This refactors and simplifies various existing code to make use of the
new function.
Reviewed-by: Mark Dilger <mark.dilger@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/
E72EAA49-354D-4C2E-8EB9-
255197F55330@enterprisedb.com