Fix pg_dump's failure to dump REPLICA IDENTITY for constraint indexes.

pg_dump knew about printing ALTER TABLE ... REPLICA IDENTITY USING INDEX
for indexes declared as indexes, but it failed to print that for indexes
declared as unique or primary-key constraints.  Per report from Achilleas
Mantzios.

This has been broken since the feature was introduced, AFAICS.
Back-patch to 9.4.

Discussion: https://postgr.es/m/1e6cc5ad-b84a-7c07-8c08-a4d0c3cdc938@matrix.gatewaynet.com