is_view seems unnecessarily slow

backend/commands/command.c has a routine is_view() that tests for
view-ness by scanning pg_rewrite (all of it) to see if the given
relation has any ON SELECT rules.

This is only used to disallow AlterTableAddConstraint and
LockTableCommand on views.  While I don't care much about the
performance of AlterTableAddConstraint, it does bug me that this
might slow down LOCK TABLE a good deal.

Any objection to replacing this routine by a test for relkind = VIEW?
        regards, tom lane