Vars hidden within a RelabelType would not be detected as compatible
with some functional dependency.  Repair by properly ignoring the
RelabelType.

Author: David Rowley
Reviewed-by: Tomas Vondra
Discussion: https://postgr.es/m/CAKJS1f-y-UEy=rsBXynBOgiW1fKMr_LVoYSGL9QOc36mLEC-ww@mail.gmail.com