Move applicable code out of RelationBuildDesc(), which nailed relations
bypass.  Non-assert builds experienced no known problems.  Back-patch to
v13, where commit c6b92041 introduced
rd_firstRelfilenodeSubid.

Kyotaro Horiguchi.  Reported by Justin Pryzby.

Discussion: https://postgr.es/m/20200907023737.GA7158@telsasoft.com