pgsql: Move EEOP_*_SYSVAR evaluation out of line.

Move EEOP_*_SYSVAR evaluation out of line.

This mainly de-duplicates code. As evaluating a system variable isn't
the hottest path and the current inline implementation ends up calling
out to an external function anyway, this is OK from a performance POV.

The main motivation for de-duplicating is the upcoming slot
abstraction work, after which there's not guaranteed to be a HeapTuple
backing the slot.

Author: Andres Freund, Amit Khandekar
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/b84a6dafbf2bb921baee53c0c1aba7719ee38817

Modified Files
--------------
src/backend/executor/execExprInterp.c | 58 ++++++++++++-----------------------
src/backend/jit/llvm/llvmjit.c | 4 +--
src/backend/jit/llvm/llvmjit_expr.c | 38 +++++------------------
src/backend/jit/llvm/llvmjit_types.c | 2 +-
src/include/executor/execExpr.h | 2 ++
src/include/jit/llvmjit.h | 2 +-
6 files changed, 33 insertions(+), 73 deletions(-)