Major Description
We infer relative `IsNotNull` constraints from logical plan's expressions in `constructIsNotNullConstraints` now. However, we don't consider the case of (nested) `Cast`.
For example:
val tr = LocalRelation('a.int, 'b.long)
val plan = tr.where('a.attr === 'b.attr).analyze
Then, the plan's constraints will have `IsNotNull(Cast(resolveColumn(tr, "a"), LongType))`, instead of `IsNotNull(resolveColumn(tr, "a"))`. This PR fixes it.
Besides, as `IsNotNull` constraints are most useful for `Attribute`, we should do recursing through any `Expression` that is null intolerant and construct `IsNotNull` constraints for all `Attribute`s under these Expressions.
For example, consider the following constraints:
val df = Seq((1,2,3)).toDF("a", "b", "c")
df.where("a + b = c").queryExecution.analyzed.constraints
The inferred isnotnull constraints should be isnotnull(a), isnotnull(b), isnotnull(c), instead of isnotnull(a + c) and isnotnull(c).
