ALTER TYPE 1: recheck index-based constraints

When ALTER TABLE rewrites a table, it reindexes, but the reindex does not
revalidate UNIQUE/EXCLUDE constraints.  This behaves badly in cases like this,
neglecting to throw an error on the new UNIQUE violation:

CREATE TABLE t (c numeric UNIQUE);
INSERT INTO t VALUES (1.1),(1.2);
ALTER TABLE t ALTER c TYPE int;

The comment gave a reason for skipping the checks: it would cause deadlocks when
we rewrite a system catalog.  So, this patch changes things to only skip the
check for system catalogs.