pgsql: Replace EEOP_DONE with special steps for return/no return

Replace EEOP_DONE with special steps for return/no return

Knowing when the side-effects of an expression is the intended result
of the execution, rather than the returnvalue, is important for being
able generate more efficient JITed code. This replaces EEOP_DONE with
two new steps: EEOP_DONE_RETURN and EEOP_DONE_NO_RETURN. Expressions
which return a value should use the former step; expressions used for
their side-effects which don't return value should use the latter.

Author: Andres Freund <andres(at)anarazel(dot)de>
Co-authored-by: Daniel Gustafsson <daniel(at)yesql(dot)se>
Reviewed-by: Andreas Karlsson <andreas(at)proxel(dot)se>
Discussion: https://postgr.es/m/415721CE-7D2E-4B74-B5D9-1950083BA03E@yesql.se
Discussion: https://postgr.es/m/20191023163849.sosqbfs5yenocez3@alap3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/8dd7c7cd0a2605d5301266a6b67a569d6a305106

Modified Files
--------------
src/backend/executor/README | 11 ++++++--
src/backend/executor/execExpr.c | 24 ++++++++--------
src/backend/executor/execExprInterp.c | 25 +++++++++++------
src/backend/executor/nodeAgg.c | 7 ++---
src/backend/jit/llvm/llvmjit_expr.c | 6 +++-
src/include/executor/execExpr.h | 7 +++--
src/include/executor/executor.h | 52 +++++++++++++++++++++++++++++++++--
7 files changed, 98 insertions(+), 34 deletions(-)