Tweak generic_type_consistency routines to avoid loss of functionality

since 7.3: 'select array_dims(histogram_bounds) from pg_stats' used to
work and still should.  Problem was that code wouldn't take input of
declared type anyarray as matching an anyarray argument.  Allow this
case as long as we don't need to determine an element type (which in
practice means as long as anyelement isn't used in the function signature).