Set scan direction appropriately for SubPlans (bug #15336)

When executing a SubPlan in an expression, the EState's direction
field was left alone, resulting in an attempt to execute the subplan
backwards if it was encountered during a backwards scan of a cursor.
Also, though much less likely, it was possible to reach the execution
of an InitPlan while in backwards-scan state.

Repair by saving/restoring estate->es_direction and forcing forward
scan mode in the relevant places.

Backpatch all the way, since this has been broken since 8.3 (prior to
commit c7ff7663e, SubPlans had their own EStates rather than sharing
the parent plan's, so there was no confusion over scan direction).

Per bug #15336 reported by Vladimir Baranoff; analysis and patch by
me, review by Tom Lane.

Discussion: https://postgr.es/m/153449812167.1304.1741624125628126322@wrigleys.postgresql.org

src/test/regress/expected/subselect.out

@@ -1137,3 +1137,20 @@ select * from (select pk,c2 from sq_limit order by c1,pk) as x limit 3;
 
 drop function explain_sq_limit();
 drop table sq_limit;
+--
+-- Ensure that backward scan direction isn't propagated into
+-- expression subqueries (bug #15336)
+--
+begin;
+declare c1 scroll cursor for
+ select * from generate_series(1,4) i
+  where i <> all (values (2),(3));
+move forward all in c1;
+fetch backward all in c1;
+ i 
+---
+ 4
+ 1
+(2 rows)
+
+commit;

src/test/regress/sql/subselect.sql

@@ -609,3 +609,19 @@ select * from (select pk,c2 from sq_limit order by c1,pk) as x limit 3;
 drop function explain_sq_limit();
 
 drop table sq_limit;
+
+--
+-- Ensure that backward scan direction isn't propagated into
+-- expression subqueries (bug #15336)
+--
+
+begin;
+
+declare c1 scroll cursor for
+ select * from generate_series(1,4) i
+  where i <> all (values (2),(3));
+
+move forward all in c1;
+fetch backward all in c1;
+
+commit;