Fix missed step in removal of useless RESULT RTEs in the planner.

Commit 4be058fe9 forgot that the append_rel_list would already be
populated at the time we remove useless result RTEs, and it might contain
PlaceHolderVars that need to be adjusted like the ones in the main parse
tree.  This could lead to "no relation entry for relid N" failures later
on, when the planner tries to do something with an unadjusted PHV.

Per report from Tom Ellis.  Back-patch to v12 where the bug came in.

Discussion: https://postgr.es/m/20201205173056.GF30712@cloudinit-builder

src/backend/optimizer/prep/prepjointree.c

@@ -3256,6 +3256,7 @@ remove_result_refs(PlannerInfo *root, int varno, Node *newjtloc)
 		subrelids = get_relids_in_jointree(newjtloc, false);
 		Assert(!bms_is_empty(subrelids));
 		substitute_phv_relids((Node *) root->parse, varno, subrelids);
+		fix_append_rel_relids(root->append_rel_list, varno, subrelids);
 	}
 
 	/*

src/test/regress/expected/join.out

@@ -3333,6 +3333,42 @@ select * from
  1 | 2 | 2
 (1 row)
 
+-- Test proper handling of appendrel PHVs during useless-RTE removal
+explain (costs off)
+select * from
+  (select 0 as z) as t1
+  left join
+  (select true as a) as t2
+  on true,
+  lateral (select true as b
+           union all
+           select a as b) as t3
+where b;
+              QUERY PLAN               
+---------------------------------------
+ Nested Loop
+   ->  Result
+   ->  Append
+         ->  Result
+         ->  Result
+               One-Time Filter: (true)
+(6 rows)
+
+select * from
+  (select 0 as z) as t1
+  left join
+  (select true as a) as t2
+  on true,
+  lateral (select true as b
+           union all
+           select a as b) as t3
+where b;
+ z | a | b 
+---+---+---
+ 0 | t | t
+ 0 | t | t
+(2 rows)
+
 --
 -- test inlining of immutable functions
 --

src/test/regress/sql/join.sql

@@ -1056,6 +1056,28 @@ select * from
   (select 1 as x) ss1 left join (select 2 as y) ss2 on (true),
   lateral (select ss2.y as z limit 1) ss3;
 
+-- Test proper handling of appendrel PHVs during useless-RTE removal
+explain (costs off)
+select * from
+  (select 0 as z) as t1
+  left join
+  (select true as a) as t2
+  on true,
+  lateral (select true as b
+           union all
+           select a as b) as t3
+where b;
+
+select * from
+  (select 0 as z) as t1
+  left join
+  (select true as a) as t2
+  on true,
+  lateral (select true as b
+           union all
+           select a as b) as t3
+where b;
+
 --
 -- test inlining of immutable functions
 --