This is useful because it gets rid of the sole direct user of
ExecAssignResultType(). A future commit will likely make use of that
and combine creating the targetlist with the initialization of the
result slot. But it seems like good code hygiene anyway.

Author: Andres Freund
Discussion: https://postgr.es/m/20170901064131.tazjxwus3k2w3ybh@alap3.anarazel.de