Suppress Append and MergeAppend plan nodes that have a single child.

If there's only one child relation, the Append or MergeAppend isn't
doing anything useful, and can be elided.  It does have a purpose
during planning though, which is to serve as a buffer between parent
and child Var numbering.  Therefore we keep it all the way through
to setrefs.c, and get rid of it only after fixing references in the
plan level(s) above it.  This works largely the same as setrefs.c's
ancient hack to get rid of no-op SubqueryScan nodes, and can even
share some code with that.

Note the change to make setrefs.c use apply_tlist_labeling rather than
ad-hoc code.  This has the effect of propagating the child's resjunk
and ressortgroupref labels, which formerly weren't propagated when
removing a SubqueryScan.  Doing that is demonstrably necessary for
the [Merge]Append cases, and seems harmless for SubqueryScan, if only
because trivial_subqueryscan is afraid to collapse cases where the
resjunk marking differs.  (I suspect that restriction could now be
removed, though it's unclear that it'd make any new matches possible,
since the outer query can't have references to a child resjunk column.)

David Rowley, reviewed by Alvaro Herrera and Tomas Vondra

Discussion: https://postgr.es/m/CAKJS1f_7u8ATyJ1JGTMHFoKDvZdeF-iEBhs+sM_SXowOr9cArg@mail.gmail.com

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -8427,17 +8427,16 @@ SELECT t1.a,t2.b,t3.c FROM fprt1 t1 INNER JOIN fprt2 t2 ON (t1.a = t2.b) INNER J
  400 | 400 | 0008
 (4 rows)
 
--- left outer join + nullable clasue
-EXPLAIN (COSTS OFF)
+-- left outer join + nullable clause
+EXPLAIN (VERBOSE, COSTS OFF)
 SELECT t1.a,t2.b,t2.c FROM fprt1 t1 LEFT JOIN (SELECT * FROM fprt2 WHERE a < 10) t2 ON (t1.a = t2.b and t1.b = t2.a) WHERE t1.a < 10 ORDER BY 1,2,3;
-                                    QUERY PLAN                                     
------------------------------------------------------------------------------------
- Sort
-   Sort Key: t1.a, ftprt2_p1.b, ftprt2_p1.c
-   ->  Append
-         ->  Foreign Scan
-               Relations: (public.ftprt1_p1 t1) LEFT JOIN (public.ftprt2_p1 fprt2)
-(5 rows)
+                                                                                                                     QUERY PLAN                                                                                                                     
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ Foreign Scan
+   Output: t1.a, ftprt2_p1.b, ftprt2_p1.c
+   Relations: (public.ftprt1_p1 t1) LEFT JOIN (public.ftprt2_p1 fprt2)
+   Remote SQL: SELECT r6.a, r9.b, r9.c FROM (public.fprt1_p1 r6 LEFT JOIN public.fprt2_p1 r9 ON (((r6.a = r9.b)) AND ((r6.b = r9.a)) AND ((r9.a < 10)))) WHERE ((r6.a < 10)) ORDER BY r6.a ASC NULLS LAST, r9.b ASC NULLS LAST, r9.c ASC NULLS LAST
+(4 rows)
 
 SELECT t1.a,t2.b,t2.c FROM fprt1 t1 LEFT JOIN (SELECT * FROM fprt2 WHERE a < 10) t2 ON (t1.a = t2.b and t1.b = t2.a) WHERE t1.a < 10 ORDER BY 1,2,3;
  a | b |  c