Compute information about EEOP_*_FETCHSOME at expression init time.
Previously this information was computed when JIT compiling an
expression. But the information is useful for assertions in the
non-JIT case too (for assertions), therefore it makes sense to move
it.
This will, in a followup commit, allow to treat different slot types
differently. E.g. for virtual slots there's no need to generate a JIT
function to deform the slot.
Author: Andres Freund
Discussion: https://postgr.es/m/
20181105210039.hh4vvi4vwoq5ba2q@alap3.anarazel.de