Teach ANALYZE to clear pg_class.relhassubclass when appropriate.

In the past, relhassubclass always remained true if a relation had ever had
child relations, even if the last subclass was long gone.  While this had
only marginal performance implications in most cases, it was annoying, and
I'm now considering some planner changes that would raise the cost of a
false positive.  It was previously impractical to fix this because of race
condition concerns.  However, given the recent change that made tablecmds.c
take ShareExclusiveLock on relations that are gaining a child (commit
fbcf4b92), we can now allow ANALYZE to
clear the flag when it's no longer relevant.  There is no additional
locking cost to do so, since ANALYZE takes ShareExclusiveLock anyway.