assuming comparison of atttypid is sufficient.  In a dropped column
atttypid will be 0, and we'd better check the physical-storage data
to make sure the tupdescs are physically compatible.
I do not believe there is a real risk before 8.0, since before that
we only used this routine to compare successive states of the tupdesc
for a particular relation.  But 8.0's typcache.c might be comparing
arbitrary tupdescs so we'd better play it safer.