[SPARK-17897][SQL] Fixed IsNotNull Constraint Inference Rule

### What changes were proposed in this pull request?
The `constraints` of an operator is the expressions that evaluate to `true` for all the rows produced. That means, the expression result should be neither `false` nor `unknown` (NULL). Thus, we can conclude that `IsNotNull` on all the constraints, which are generated by its own predicates or propagated from the children. The constraint can be a complex expression. For better usage of these constraints, we try to push down `IsNotNull` to the lowest-level expressions (i.e., `Attribute`). `IsNotNull` can be pushed through an expression when it is null intolerant. (When the input is NULL, the null-intolerant expression always evaluates to NULL.)

Below is the existing code we have for `IsNotNull` pushdown.
```Scala
  private def scanNullIntolerantExpr(expr: Expression): Seq[Attribute] = expr match {
    case a: Attribute => Seq(a)
    case _: NullIntolerant | IsNotNull(_: NullIntolerant) =>
      expr.children.flatMap(scanNullIntolerantExpr)
    case _ => Seq.empty[Attribute]
  }
```

**`IsNotNull` itself is not null-intolerant.** It converts `null` to `false`. If the expression does not include any `Not`-like expression, it works; otherwise, it could generate a wrong result. This PR is to fix the above function by removing the `IsNotNull` from the inference. After the fix, when a constraint has a `IsNotNull` expression, we infer new attribute-specific `IsNotNull` constraints if and only if `IsNotNull` appears in the root.

Without the fix, the following test case will return empty.
```Scala
val data = Seq[java.lang.Integer](1, null).toDF("key")
data.filter("not key is not null").show()
```
Before the fix, the optimized plan is like
```
== Optimized Logical Plan ==
Project [value#1 AS key#3]
+- Filter (isnotnull(value#1) && NOT isnotnull(value#1))
   +- LocalRelation [value#1]
```

After the fix, the optimized plan is like
```
== Optimized Logical Plan ==
Project [value#1 AS key#3]
+- Filter NOT isnotnull(value#1)
   +- LocalRelation [value#1]
```

### How was this patch tested?
Added a test

Author: gatorsmile <gatorsmile@gmail.com>

Closes #16067 from gatorsmile/isNotNull2.

(cherry picked from commit 2eb093de)
Signed-off-by: Wenchen Fan <wenchen@databricks.com>