pgsql: Simplify executor's handling of CaseTestExpr & CoerceToDomainVal

Simplify executor's handling of CaseTestExpr & CoerceToDomainValue.

Instead of deciding at runtime whether to read from casetest.value
or caseValue_datum, split EEOP_CASE_TESTVAL into two opcodes and
make the decision during expression compilation. Similarly for
EEOP_DOMAIN_TESTVAL. This actually results in net less code,
mainly because llvmjit_expr.c's code for handling these opcodes
gets shorter. The performance gain is doubtless negligible, but
this seems worth changing anyway on grounds of simplicity and
understandability.

Author: Andreas Karlsson <andreas(at)proxel(dot)se>
Co-authored-by: Xing Guo <higuoxing(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/CACpMh+AiBYAWn+D1aU7Rsy-V1tox06Cbc0H3qA7rwL5zdJ=anQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/115a365519bfd53a65bf17d253b26902eff0c337

Modified Files
--------------
src/backend/executor/execExpr.c | 34 ++++++++++++--------
src/backend/executor/execExprInterp.c | 59 ++++++++++++++---------------------
src/backend/jit/llvm/llvmjit_expr.c | 56 ++++++++++-----------------------
src/include/executor/execExpr.h | 2 ++
4 files changed, 63 insertions(+), 88 deletions(-)