Adjust psql describe queries so that any pg_foo_is_visible() condition

Adjust psql describe queries so that any pg_foo_is_visible() condition
is applied last, after other constraints such as name patterns.  This
is useful first because the pg_foo_is_visible() functions are relatively
expensive, and second because it minimizes the prospects for race
conditions.  The change is fragile though since it makes unwarranted
assumptions about planner behavior, ie, that WHERE clauses will be
executed in the original order if there's not reason to change it.
This should fix ... or at least hide ... an intermittent failure in the
prepared_xacts regression test, while we think about what else to do.