An upcoming patch which changes how inheritance planning works requires
adding a new function that does a similar job to set_append_rel_size() but
for child target relations.  To save it from having to duplicate the qual
building code, move that to a separate function first.

Here we also change things so that we never attempt to build security quals
after detecting some const false child quals.  We needlessly used to do this
just before we marked the child relation as a dummy rel.

In passing, this also moves the partition pruned check to before the qual
building code.  We don't need to build the child quals before we check if
the partition has been pruned.

Author: David Rowley
Discussion: https://postgr.es/m/CAKJS1f_i+jrrD+if8qC7KPuTAAWsd=dtepgY_7u=P86GDEwm7A@mail.gmail.com