pgsql: Compute information about EEOP_*_FETCHSOME at expression initti

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/675af5c01e297262cd20d7416f7e568393c22c6e

Modified Files
--------------
src/backend/executor/execExpr.c     | 104 ++++++++++++++++++++++++++++++++++++
src/backend/jit/llvm/llvmjit_expr.c |  44 ++++++---------
src/include/executor/execExpr.h     |   5 ++
3 files changed, 124 insertions(+), 29 deletions(-)