pgsql: Fix "SELECT simple_expression INTO" for expression becoming non-

Fix "SELECT simple_expression INTO" for expression becoming non-simple.

Commit ce8d5fe0e failed to think hard enough about the case where
we're forced to replan the expression and it becomes no longer simple.
The code produced the right expression result, but its handling of
FOUND and ROW_COUNT was too simplistic since we might have obtained
zero rows. To fix, don't use exec_eval_expr, but go directly to
exec_eval_simple_expr, and if that reports failure then fall through
to the full SPI code path.

Another thinko in ce8d5fe0e is that it added

if (expr->plan == NULL)
exec_prepare_plan(estate, expr, 0);

which is quite useless since we cannot reach here without having
planned the expression at least once. (If the call were reachable
it'd likely be wrong, since it's not passing the same flags as
either the preceding call or the one inside exec_eval_expr.)

Author: shihao zhong <zhong950419(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/CAGRkXqQB_aqWU9OeVPGvATBVZ7c9EYrNXJyHG0jKXW0FGAy2-g@mail.gmail.com
Backpatch-through: 19

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e3199f3149e77d42ef799858ba3f8b343db09def

Modified Files
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src/pl/plpgsql/src/pl_exec.c | 45 +++++++++++++++++++++++++-------------------
1 file changed, 26 insertions(+), 19 deletions(-)