Have the planner replace COUNT(ANY) with COUNT(*), when possible

This adds SupportRequestSimplifyAggref to allow pg_proc.prosupport
functions to receive an Aggref and allow them to determine if there is a
way that the Aggref call can be optimized.

Also added is a support function to allow transformation of COUNT(ANY)
into COUNT(*).  This is possible to do when the given "ANY" cannot be
NULL and also that there are no ORDER BY / DISTINCT clauses within the
Aggref.  This is a useful transformation to do as it is common that
people write COUNT(1), which until now has added unneeded overhead.
When counting a NOT NULL column.  The overheads can be worse as that
might mean deforming more of the tuple, which for large fact tables may
be many columns in.

It may be possible to add prosupport functions for other aggregates.  We
could consider if ORDER BY could be dropped for some calls, e.g. the
ORDER BY is quite useless in MAX(c ORDER BY c).

There is a little bit of passing fallout from adjusting
expr_is_nonnullable() to handle Const which results in a plan change in
the aggregates.out regression test.  Previously, nothing was able to
determine that "One-Time Filter: (100 IS NOT NULL)" was always true,
therefore useless to include in the plan.

Author: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: Corey Huinker <corey.huinker@gmail.com>
Reviewed-by: Matheus Alcantara <matheusssilv97@gmail.com>
Discussion: https://postgr.es/m/CAApHDvqGcPTagXpKfH=CrmHBqALpziThJEDs_MrPqjKVeDF9wA@mail.gmail.com

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -2975,9 +2975,9 @@ select sum(t1.c1), count(t2.c1) from ft1 t1 inner join ft2 t2 on (t1.c1 = t2.c1)
                                                          QUERY PLAN                                                         
 ----------------------------------------------------------------------------------------------------------------------------
  Aggregate
-   Output: sum(t1.c1), count(t2.c1)
+   Output: sum(t1.c1), count(*)
    ->  Foreign Scan
-         Output: t1.c1, t2.c1
+         Output: t1.c1
          Filter: (((((t1.c1 * t2.c1) / (t1.c1 * t2.c1)))::double precision * random()) <= '1'::double precision)
          Relations: (public.ft1 t1) INNER JOIN (public.ft2 t2)
          Remote SQL: SELECT r1."C 1", r2."C 1" FROM ("S 1"."T 1" r1 INNER JOIN "S 1"."T 1" r2 ON (((r2."C 1" = r1."C 1"))))
@@ -3073,12 +3073,12 @@ select c2 * (random() <= 1)::int as c2 from ft2 group by c2 * (random() <= 1)::i
 -- GROUP BY clause in various forms, cardinal, alias and constant expression
 explain (verbose, costs off)
 select count(c2) w, c2 x, 5 y, 7.0 z from ft1 group by 2, y, 9.0::int order by 2;
-                                                 QUERY PLAN                                                 
-------------------------------------------------------------------------------------------------------------
+                                                QUERY PLAN                                                 
+-----------------------------------------------------------------------------------------------------------
  Foreign Scan
-   Output: (count(c2)), c2, 5, 7.0, 9
+   Output: (count(*)), c2, 5, 7.0, 9
    Relations: Aggregate on (public.ft1)
-   Remote SQL: SELECT count(c2), c2, 5, 7.0, 9 FROM "S 1"."T 1" GROUP BY 2, 3, 5 ORDER BY c2 ASC NULLS LAST
+   Remote SQL: SELECT count(*), c2, 5, 7.0, 9 FROM "S 1"."T 1" GROUP BY 2, 3, 5 ORDER BY c2 ASC NULLS LAST
 (4 rows)
 
 select count(c2) w, c2 x, 5 y, 7.0 z from ft1 group by 2, y, 9.0::int order by 2;
@@ -3379,8 +3379,8 @@ select distinct (select count(*) filter (where t2.c2 = 6 and t2.c1 < 10) from ft
 -- Inner query is aggregation query
 explain (verbose, costs off)
 select distinct (select count(t1.c1) filter (where t2.c2 = 6 and t2.c1 < 10) from ft1 t1 where t1.c1 = 6) from ft2 t2 where t2.c2 % 6 = 0 order by 1;
-                                                                      QUERY PLAN                                                                      
-------------------------------------------------------------------------------------------------------------------------------------------------------
+                                                                    QUERY PLAN                                                                    
+--------------------------------------------------------------------------------------------------------------------------------------------------
  Unique
    Output: ((SubPlan expr_1))
    ->  Sort
@@ -3391,9 +3391,9 @@ select distinct (select count(t1.c1) filter (where t2.c2 = 6 and t2.c1 < 10) fro
                Remote SQL: SELECT "C 1", c2 FROM "S 1"."T 1" WHERE (((c2 % 6) = 0))
                SubPlan expr_1
                  ->  Foreign Scan
-                       Output: (count(t1.c1) FILTER (WHERE ((t2.c2 = 6) AND (t2.c1 < 10))))
+                       Output: (count(*) FILTER (WHERE ((t2.c2 = 6) AND (t2.c1 < 10))))
                        Relations: Aggregate on (public.ft1 t1)
-                       Remote SQL: SELECT count("C 1") FILTER (WHERE (($1::integer = 6) AND ($2::integer < 10))) FROM "S 1"."T 1" WHERE (("C 1" = 6))
+                       Remote SQL: SELECT count(*) FILTER (WHERE (($1::integer = 6) AND ($2::integer < 10))) FROM "S 1"."T 1" WHERE (("C 1" = 6))
 (13 rows)
 
 select distinct (select count(t1.c1) filter (where t2.c2 = 6 and t2.c1 < 10) from ft1 t1 where t1.c1 = 6) from ft2 t2 where t2.c2 % 6 = 0 order by 1;