[SPARK-13869][SQL] Remove redundant conditions while combining filters

## What changes were proposed in this pull request?

**[I'll link it to the JIRA once ASF JIRA is back online]**

This PR modifies the existing `CombineFilters` rule to remove redundant conditions while combining individual filter predicates. For instance, queries of the form `table.where('a === 1 && 'b === 1).where('a === 1 && 'c === 1)` will now be optimized to ` table.where('a === 1 && 'b === 1 && 'c === 1)` (instead of ` table.where('a === 1 && 'a === 1 && 'b === 1 && 'c === 1)`)

## How was this patch tested?

Unit test in `FilterPushdownSuite`

Author: Sameer Agarwal <sameer@databricks.com>

Closes apache#11670 from sameeragarwal/combine-filters.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala

@@ -818,12 +818,19 @@ object CombineUnions extends Rule[LogicalPlan] {
 }
 
 /**
- * Combines two adjacent [[Filter]] operators into one, merging the
- * conditions into one conjunctive predicate.
+ * Combines two adjacent [[Filter]] operators into one, merging the non-redundant conditions into
+ * one conjunctive predicate.
  */
-object CombineFilters extends Rule[LogicalPlan] {
+object CombineFilters extends Rule[LogicalPlan] with PredicateHelper {
   def apply(plan: LogicalPlan): LogicalPlan = plan transform {
-    case ff @ Filter(fc, nf @ Filter(nc, grandChild)) => Filter(And(nc, fc), grandChild)
+    case ff @ Filter(fc, nf @ Filter(nc, grandChild)) =>
+      (ExpressionSet(splitConjunctivePredicates(fc)) --
+        ExpressionSet(splitConjunctivePredicates(nc))).reduceOption(And) match {
+        case Some(ac) =>
+          Filter(And(ac, nc), grandChild)
+        case None =>
+          nf
+      }
   }
 }
 

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

@@ -81,6 +81,21 @@ class FilterPushdownSuite extends PlanTest {
     comparePlans(optimized, correctAnswer)
   }
 
+  test("combine redundant filters") {
+    val originalQuery =
+      testRelation
+        .where('a === 1 && 'b === 1)
+        .where('a === 1 && 'c === 1)
+
+    val optimized = Optimize.execute(originalQuery.analyze)
+    val correctAnswer =
+      testRelation
+        .where('a === 1 && 'b === 1 && 'c === 1)
+        .analyze
+
+    comparePlans(optimized, correctAnswer)
+  }
+
   test("can't push without rewrite") {
     val originalQuery =
       testRelation