[SPARK-25368][SQL] Incorrect predicate pushdown returns wrong result

## What changes were proposed in this pull request?
How to reproduce:
```scala
val df1 = spark.createDataFrame(Seq(
   (1, 1)
)).toDF("a", "b").withColumn("c", lit(null).cast("int"))
val df2 = df1.union(df1).withColumn("d", spark_partition_id).filter($"c".isNotNull)
df2.show

+---+---+----+---+
|  a|  b|   c|  d|
+---+---+----+---+
|  1|  1|null|  0|
|  1|  1|null|  1|
+---+---+----+---+
```
`filter($"c".isNotNull)` was transformed to `(null <=> c#10)` before #19201, but it is transformed to `(c#10 = null)` since #20155. This pr revert it to `(null <=> c#10)` to fix this issue.

## How was this patch tested?

unit tests

Closes #22368 from wangyum/SPARK-25368.

Authored-by: Yuming Wang <yumwang@ebay.com>
Signed-off-by: gatorsmile <gatorsmile@gmail.com>

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/LogicalPlan.scala

@@ -159,7 +159,7 @@ abstract class UnaryNode extends LogicalPlan {
     var allConstraints = child.constraints.asInstanceOf[Set[Expression]]
     projectList.foreach {
       case a @ Alias(l: Literal, _) =>
-        allConstraints += EqualTo(a.toAttribute, l)
+        allConstraints += EqualNullSafe(a.toAttribute, l)
       case a @ Alias(e, _) =>
         // For every alias in `projectList`, replace the reference in constraints by its attribute.
         allConstraints ++= allConstraints.map(_ transform {

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/InferFiltersFromConstraintsSuite.scala

@@ -196,7 +196,7 @@ class InferFiltersFromConstraintsSuite extends PlanTest {
 
   test("constraints should be inferred from aliased literals") {
     val originalLeft = testRelation.subquery('left).as("left")
-    val optimizedLeft = testRelation.subquery('left).where(IsNotNull('a) && 'a === 2).as("left")
+    val optimizedLeft = testRelation.subquery('left).where(IsNotNull('a) && 'a <=> 2).as("left")
 
     val right = Project(Seq(Literal(2).as("two")), testRelation.subquery('right)).as("right")
     val condition = Some("left.a".attr === "right.two".attr)

sql/core/src/test/scala/org/apache/spark/sql/DataFrameSuite.scala

@@ -2552,4 +2552,21 @@ class DataFrameSuite extends QueryTest with SharedSQLContext {
     }
   }
 
+  test("SPARK-25368 Incorrect predicate pushdown returns wrong result") {
+    def check(newCol: Column, filter: Column, result: Seq[Row]): Unit = {
+      val df1 = spark.createDataFrame(Seq(
+        (1, 1)
+      )).toDF("a", "b").withColumn("c", newCol)
+
+      val df2 = df1.union(df1).withColumn("d", spark_partition_id).filter(filter)
+      checkAnswer(df2, result)
+    }
+
+    check(lit(null).cast("int"), $"c".isNull, Seq(Row(1, 1, null, 0), Row(1, 1, null, 1)))
+    check(lit(null).cast("int"), $"c".isNotNull, Seq())
+    check(lit(2).cast("int"), $"c".isNull, Seq())
+    check(lit(2).cast("int"), $"c".isNotNull, Seq(Row(1, 1, 2, 0), Row(1, 1, 2, 1)))
+    check(lit(2).cast("int"), $"c" === 2, Seq(Row(1, 1, 2, 0), Row(1, 1, 2, 1)))
+    check(lit(2).cast("int"), $"c" =!= 2, Seq())
+  }
 }