pgsql: Marginal speedup in RelationIsVisible and TypeIsVisible: avoid a

Log Message:
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Marginal speedup in RelationIsVisible and TypeIsVisible: avoid a redundant
cache lookup in the success case. This won't help much for cases where
the given relation is far down the search path, but it does not hurt in
any cases either; and it requires only a little new code. Per gripe from
Jim Nasby about slowness of \d with many tables.

Modified Files:
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pgsql/src/backend/catalog:
namespace.c (r1.77 -> r1.78)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/namespace.c.diff?r1=1.77&r2=1.78)