planner: remove correlated column sort items (#9435)

[zz-jason]

cherry pick #9435 to release 2.1

[zz-jason]

/run-all-tests tikv=release-2.1 pd=release-2.1 tidb-test=release-2.1

[XuHuaiyu]

2019/02/25 14:55:34.434 main.go:688: [fatal] run test [topn_push_down] err: sql:desc select * from t1 where t1.a in (select t2.a as a from t2 where t2.b > t1.b order by t1.b limit 1);: run "desc select * from t1 where t1.a in (select t2.a as a from t2 where t2.b > t1.b order by t1.b limit 1);" at line 183 err, we need:

desc select * from t1 where t1.a in (select t2.a as a from t2 where t2.b > t1.b order by t1.b limit 1);

id	count	task	operator info

Apply_15	9990.00	root	semi join, inner:Selection_19, equal:[eq(test.t1.a, test.t2.a)]

├─TableReader_18	9990.00	root	data:Selection_17

│ └─Selection_17	9990.00	cop	not(isnull(test.t1.a))

│   └─TableScan_16	10000.00	cop	table:t1, range:[-inf,+inf], keep order:false, stats:pseudo

└─Selection_19	0.80	root	not(isnull(test.t2.a))

  └─Limit_20	1.00	root	offset:0, count:1

    └─TableReader_26	1.00	root	data:Limit_25

      └─Limit_25	1.00	cop	offset:0, count:1

        └─Selection_24	1.00	cop

but got:

desc select * from t1 where t1.a in (select t2.a as a from t2 where t2.b > t1.b order by t1.b limit 1);

id	count	task	operator info

Apply_14	10000.00	root	semi join, inner:Limit_17, equal:[eq(test.t1.a, test.t2.a)]

├─TableReader_16	10000.00	root	data:TableScan_15

│ └─TableScan_15	10000.00	cop	table:t1, range:[-inf,+inf], keep order:false, stats:pseudo

└─Limit_17	1.00	root	offset:0, count:1

  └─TableReader_23	1.00	root	data:Limit_22

    └─Limit_22	1.00	cop	offset:0, count:1

      └─Selection_21	1.00	cop	gt(test.t2.b, test.t1.b)

        └─TableScan_20	1.25	cop	table:t2, range:[-inf,+inf], keep order:false, stats:pseudo

[zz-jason]

/run-all-tests tikv=release-2.1 pd=release-2.1 tidb-test=release-2.1

[XuHuaiyu]

LGTM

[alivxxx]

LGTM