pgsql: Found the problem with my operator-family changes: by fetching

Log Message:
-----------
Found the problem with my operator-family changes: by fetching from
pg_opclass during LookupOpclassInfo(), I'd turned pg_opclass_oid_index
into a critical system index.  However the problem could only manifest
during a backend's first attempt to load opclass data, and then only
if it had successfully loaded pg_internal.init and subsequently received
a relcache flush; which made it impossible to reproduce in sequential
tests and darn hard even in parallel tests.  Memo to self: when
exercising cache flush scenarios, must disable LookupOpclassInfo's
internal cache too.

Modified Files:
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    pgsql/src/backend/utils/cache:
        relcache.c (r1.251 -> r1.252)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/relcache.c.diff?r1=1.251&r2=1.252)