Use lfirst_node() and linitial_node() where appropriate in planner.c.

There's no particular reason to target this module for the first
wholesale application of these macros; but we gotta start somewhere.

Ashutosh Bapat and Jeevan Chalke

Discussion: https://postgr.es/m/CAFjFpRcNr3r=u0ni=7A4GD9NnHQVq+dkFafzqo2rS6zy=dt1eg@mail.gmail.com