Undo faulty attempt at not relying on RINFO_IS_PUSHED_DOWN.

I've had a bee in my bonnet for some time about getting rid of
RestrictInfo.is_pushed_down, because it's squishily defined and
requires not-inexpensive extra tests to use (cf RINFO_IS_PUSHED_DOWN).
In commit 2489d76c4, I tried to make remove_rel_from_query() not
depend on that macro; but the replacement test is buggy,
as exposed by a report from Rushabh Lathia and Robert Haas.
That change was pretty incidental to the main goal of 2489d76c4,
so let's just revert it for now.  (Getting rid of is_pushed_down
is still far away, anyway.)

Discussion: https://postgr.es/m/CA+TgmoYco=hmg+iX1CW9Y1_CzNoSL81J03wUG-d2_3=rue+L2A@mail.gmail.com

src/backend/optimizer/plan/analyzejoins.c

@@ -466,12 +466,7 @@ remove_rel_from_query(PlannerInfo *root, int relid, int ojrelid,
 
 		remove_join_clause_from_rels(root, rinfo, rinfo->required_relids);
 
-		/*
-		 * If the qual lists ojrelid in its required_relids, it must have come
-		 * from above the outer join we're removing (so we need to keep it);
-		 * if it does not, then it didn't and we can discard it.
-		 */
-		if (bms_is_member(ojrelid, rinfo->required_relids))
+		if (RINFO_IS_PUSHED_DOWN(rinfo, joinrelids))
 		{
 			/*
 			 * There might be references to relid or ojrelid in the

src/test/regress/expected/join.out

@@ -5320,6 +5320,22 @@ select 1 from (select a.id FROM a left join b on a.b_id = b.id) q,
          Filter: (a.id = i)
 (4 rows)
 
+-- check join removal within RHS of an outer join
+explain (costs off)
+select c.id, ss.a from c
+  left join (select d.a from onerow, d left join b on d.a = b.id) ss
+  on c.id = ss.a;
+           QUERY PLAN           
+--------------------------------
+ Hash Right Join
+   Hash Cond: (d.a = c.id)
+   ->  Nested Loop
+         ->  Seq Scan on onerow
+         ->  Seq Scan on d
+   ->  Hash
+         ->  Seq Scan on c
+(7 rows)
+
 CREATE TEMP TABLE parted_b (id int PRIMARY KEY) partition by range(id);
 CREATE TEMP TABLE parted_b1 partition of parted_b for values from (0) to (10);
 -- test join removals on a partitioned table

src/test/regress/sql/join.sql

@@ -1927,6 +1927,12 @@ explain (costs off)
 select 1 from (select a.id FROM a left join b on a.b_id = b.id) q,
 			  lateral generate_series(1, q.id) gs(i) where q.id = gs.i;
 
+-- check join removal within RHS of an outer join
+explain (costs off)
+select c.id, ss.a from c
+  left join (select d.a from onerow, d left join b on d.a = b.id) ss
+  on c.id = ss.a;
+
 CREATE TEMP TABLE parted_b (id int PRIMARY KEY) partition by range(id);
 CREATE TEMP TABLE parted_b1 partition of parted_b for values from (0) to (10);