BUG #15001: planner cann't distinguish composite index?

The following bug has been logged on the website:

Bug reference:      15001
Logged by:          Zhou Digoal
Email address:      digoal@126.com
PostgreSQL version: 10.1
Operating system:   centos 7.x x64
Description:

```
postgres=# create table tbl(c1 int, c2 int, c3 int);
CREATE TABLE
postgres=# create index idx_tbl on tbl (c1,c2);
CREATE INDEX
postgres=# insert into tbl select random()*100, random()*10 from
generate_series(1,10000000);
INSERT 0 10000000

postgres=# explain select c1,c2 , count(*) from tbl group by c2,c1;
                                                     QUERY PLAN
                                       
--------------------------------------------------------------------------------------------------------------------
 Finalize GroupAggregate  (cost=176259.41..176337.18 rows=1111 width=16)
   Group Key: c2, c1
   ->  Sort  (cost=176259.41..176276.08 rows=6666 width=16)
         Sort Key: c2, c1
         ->  Gather  (cost=175158.32..175836.03 rows=6666 width=16)
               Workers Planned: 6
               ->  Partial HashAggregate  (cost=174158.32..174169.43
rows=1111 width=16)
                     Group Key: c2, c1
                     ->  Parallel Index Only Scan using idx_tbl on tbl
(cost=0.43..161658.26 rows=1666675 width=8)
(9 rows)

postgres=# explain select c1,c2 , count(*) from tbl group by c1,c2;
                                                  QUERY PLAN
                                 
--------------------------------------------------------------------------------------------------------------
 Finalize GroupAggregate  (cost=1000.53..176040.80 rows=1111 width=16)
   Group Key: c1, c2
   ->  Gather Merge  (cost=1000.53..175979.69 rows=6666 width=16)
         Workers Planned: 6
         ->  Partial GroupAggregate  (cost=0.43..174169.43 rows=1111
width=16)
               Group Key: c1, c2
               ->  Parallel Index Only Scan using idx_tbl on tbl
(cost=0.43..161658.26 rows=1666675 width=8)
(7 rows)
```

i need to set enable_sort=off, so planner can choose the same planner with
c1,c2 and c2,c1 group by. 

```
postgres=# set enable_sort=off;
SET
postgres=# explain select c1,c2 , count(*) from tbl group by c2,c1;
                                                  QUERY PLAN
                                 
--------------------------------------------------------------------------------------------------------------
 Finalize HashAggregate  (cost=175886.03..175897.14 rows=1111 width=16)
   Group Key: c2, c1
   ->  Gather  (cost=175158.32..175836.03 rows=6666 width=16)
         Workers Planned: 6
         ->  Partial HashAggregate  (cost=174158.32..174169.43 rows=1111
width=16)
               Group Key: c2, c1
               ->  Parallel Index Only Scan using idx_tbl on tbl
(cost=0.43..161658.26 rows=1666675 width=8)
(7 rows)
```