jit: Remove redundancies in expression evaluation code generation.
This merges the code emission for a number of opcodes by handling the
behavioural difference more locally. This reduces code, and also
improves the generated code a bit in some cases, by removing redundant
constants.
Author: Andres Freund
Discussion: https://postgr.es/m/
20191023163849.sosqbfs5yenocez3@alap3.anarazel.de