expression eval, jit: Minor code cleanups.
This mostly consists of using C99 style for loops, moving variables
into narrower scopes, and a smattering of other minor improvements.
Done separately to make it easier to review patches with actual
functional changes.
Author: Andres Freund
Discussion: https://postgr.es/m/
20191023163849.sosqbfs5yenocez3@alap3.anarazel.de