Cannot simplify the predicate in the Selection

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

run this test case in the ```tests/integrationtest/t/planner/core/casetest/predicate_simplification.test```

```
select* from IDT_MC21780 t1 where t1.col1 < "1976-09-09 20:21:11" and t1.col1 in ("2021-07-14 09:28:16", "1982-01-09 03:36:39", "1970-12-18 10:53:28");
```

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

```
eq(planner__core__casetest__predicate_simplification.idt_mc21780.col1, 1970-12-18 10:53:28.000000)
```

### 3. What did you see instead (Required)

```
└─Selection 10.00 cop[tikv] eq(planner__core__casetest__predicate_simplification.idt_mc21780.col1, 1970-12-18 10:53:28.000000), lt(planner__core__casetest__predicate_simplification.idt_mc21780.col1, 1976-09-09 20:21:11.000000)
```

its' condition is ```eq(planner__core__casetest__predicate_simplification.idt_mc21780.col1, 1970-12-18 10:53:28.000000), lt(planner__core__casetest__predicate_simplification.idt_mc21780.col1, 1976-09-09 20:21:11.000000)```
### 4. What is your TiDB version? (Required)

[254849d](https://github.com/pingcap/tidb/commit/254849d23a181c9f88f780eba34678b808a625b9)