pgsql: Perform slot validity checks in a separate pass over expression.

Perform slot validity checks in a separate pass over expression.

This reduces code duplication a bit, but the primary benefit that it
makes JITing expression evaluation easier. When doing so we can't, as
previously done in the interpreted case, really change opcode without
recompiling. Nor dow we just carry around unnecessary branches to
avoid re-checking over and over.

As a minor side-effect this makes ExecEvalStepOp() O(log(N)) rather
than O(N).

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b40933101ca622aa8a35b6fe07ace36effadf1c7

Modified Files
--------------
src/backend/executor/execExpr.c | 7 +-
src/backend/executor/execExprInterp.c | 277 ++++++++++++++++++----------------
src/include/executor/execExpr.h | 14 +-
src/include/nodes/execnodes.h | 3 +
4 files changed, 165 insertions(+), 136 deletions(-)