If a field is incompressible ('compressed' data is actually larger than

If a field is incompressible ('compressed' data is actually larger than
source, due to addition of header overhead), store it as plain data
rather than pseudo-compressed data.  This saves a few microseconds when
reading it out, but much more importantly guarantees that the toaster
won't actually expand tuples that contain incompressible data.  That's
essential to avoid 'Tuple too big' failures with large objects.