Shouldn't push down the topN when the result for the order by item is uncertain

[Reminiscent]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

drop table if exists t3;
CREATE TABLE t3(c0 INT, primary key(c0));
insert into t3 values(1), (2), (3), (4), (5);
SELECT v2.c0 FROM (select rand() as c0 from t3) v2 order by v2.c0 limit 100;

2. What did you expect to see? (Required)

The result should be ordered.

mysql> SELECT v2.c0 FROM (select rand() as c0 from t3) v2 order by v2.c0 limit 100;
+---------------------+
| c0                  |
+---------------------+
| 0.10052517065827285 |
|  0.7717371729870673 |
|  0.8531838886935114 |
|   0.883147331777166 |
|  0.9643439622263833 |
+---------------------+
5 rows in set (0.00 sec)

3. What did you see instead (Required)

The result is not ordered.

mysql> SELECT v2.c0 FROM (select rand() as c0 from t3) v2 order by v2.c0 limit 100;
+--------------------+
| c0                 |
+--------------------+
| 0.3638526968321322 |
| 0.3650706674578326 |
| 0.7337952775264115 |
| 0.5737644159922045 |
| 0.6674362781154329 |
+--------------------+
5 rows in set (0.00 sec)

mysql> explain SELECT v2.c0 FROM (select rand() as c0 from t3) v2 order by v2.c0 limit 2;
+--------------------------------+----------+-----------+---------------+--------------------------------+
| id                             | estRows  | task      | access object | operator info                  |
+--------------------------------+----------+-----------+---------------+--------------------------------+
| Projection_7                   | 2.00     | root      |               | rand()->Column#2               |
| └─Projection_13                | 2.00     | root      |               | test.t3.c0                     |
|   └─TopN_8                     | 2.00     | root      |               | Column#3, offset:0, count:2    |
|     └─Projection_14            | 10000.00 | root      |               | test.t3.c0, rand()->Column#3   |
|       └─TableReader_12         | 10000.00 | root      |               | data:TableFullScan_11          |
|         └─TableFullScan_11     | 10000.00 | cop[tikv] | table:t3      | keep order:false, stats:pseudo |
+--------------------------------+----------+-----------+---------------+--------------------------------+
6 rows in set (0.00 sec)

4. What is your TiDB version? (Required)

latest master