Use the new castNode() macro in a number of places.
This is far from a pervasive conversion, but it's a good starting
point.
Author: Peter Eisentraut, with some minor changes by me
Reviewed-By: Tom Lane
Discussion: https://postgr.es/m/
c5d387d9-3440-f5e0-f9d4-
71d53b9fbe52@2ndquadrant.com