[SQL] Cleaned up ConstantFolding slightly.

Moved couple rules out of NullPropagation and added more comments.

Author: Reynold Xin <rxin@apache.org>

Closes #1430 from rxin/sql-folding-rule and squashes the following commits:

7f9a197 [Reynold Xin] Updated documentation for ConstantFolding.
7f8cf61 [Reynold Xin] [SQL] Cleaned up ConstantFolding slightly.

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

@@ -153,11 +153,13 @@ object NullPropagation extends Rule[LogicalPlan] {
       case e @ GetItem(Literal(null, _), _) => Literal(null, e.dataType)
       case e @ GetItem(_, Literal(null, _)) => Literal(null, e.dataType)
       case e @ GetField(Literal(null, _), _) => Literal(null, e.dataType)
-      case e @ Coalesce(children) => {
-        val newChildren = children.filter(c => c match {
+
+      // For Coalesce, remove null literals.
+      case e @ Coalesce(children) =>
+        val newChildren = children.filter {
           case Literal(null, _) => false
           case _ => true
-        })
+        }
         if (newChildren.length == 0) {
           Literal(null, e.dataType)
         } else if (newChildren.length == 1) {
@@ -165,15 +167,11 @@ object NullPropagation extends Rule[LogicalPlan] {
         } else {
           Coalesce(newChildren)
         }
-      }
-      case e @ If(Literal(v, _), trueValue, falseValue) => if (v == true) trueValue else falseValue
-      case e @ In(Literal(v, _), list) if (list.exists(c => c match {
-          case Literal(candidate, _) if candidate == v => true
-          case _ => false
-        })) => Literal(true, BooleanType)
+
       case e @ Substring(Literal(null, _), _, _) => Literal(null, e.dataType)
       case e @ Substring(_, Literal(null, _), _) => Literal(null, e.dataType)
       case e @ Substring(_, _, Literal(null, _)) => Literal(null, e.dataType)
+
       // Put exceptional cases above if any
       case e: BinaryArithmetic => e.children match {
         case Literal(null, _) :: right :: Nil => Literal(null, e.dataType)
@@ -201,9 +199,19 @@ object NullPropagation extends Rule[LogicalPlan] {
 object ConstantFolding extends Rule[LogicalPlan] {
   def apply(plan: LogicalPlan): LogicalPlan = plan transform {
     case q: LogicalPlan => q transformExpressionsDown {
-      // Skip redundant folding of literals.
+      // Skip redundant folding of literals. This rule is technically not necessary. Placing this
+      // here avoids running the next rule for Literal values, which would create a new Literal
+      // object and running eval unnecessarily.
       case l: Literal => l
+
+      // Fold expressions that are foldable.
       case e if e.foldable => Literal(e.eval(null), e.dataType)
+
+      // Fold "literal in (item1, item2, ..., literal, ...)" into true directly.
+      case In(Literal(v, _), list) if list.exists {
+          case Literal(candidate, _) if candidate == v => true
+          case _ => false
+        } => Literal(true, BooleanType)
     }
   }
 }
@@ -233,6 +241,9 @@ object BooleanSimplification extends Rule[LogicalPlan] {
           case (l, Literal(false, BooleanType)) => l
           case (_, _) => or
         }
+
+      // Turn "if (true) a else b" into "a", and if (false) a else b" into "b".
+      case e @ If(Literal(v, _), trueValue, falseValue) => if (v == true) trueValue else falseValue
     }
   }
 }
@@ -254,12 +265,12 @@ object CombineFilters extends Rule[LogicalPlan] {
  */
 object SimplifyFilters extends Rule[LogicalPlan] {
   def apply(plan: LogicalPlan): LogicalPlan = plan transform {
-    case Filter(Literal(true, BooleanType), child) =>
-      child
-    case Filter(Literal(null, _), child) =>
-      LocalRelation(child.output)
-    case Filter(Literal(false, BooleanType), child) =>
-      LocalRelation(child.output)
+    // If the filter condition always evaluate to true, remove the filter.
+    case Filter(Literal(true, BooleanType), child) => child
+    // If the filter condition always evaluate to null or false,
+    // replace the input with an empty relation.
+    case Filter(Literal(null, _), child) => LocalRelation(child.output, data = Seq.empty)
+    case Filter(Literal(false, BooleanType), child) => LocalRelation(child.output, data = Seq.empty)
   }
 }
 
@@ -301,7 +312,7 @@ object PushPredicateThroughJoin extends Rule[LogicalPlan] with PredicateHelper {
   /**
    * Splits join condition expressions into three categories based on the attributes required
    * to evaluate them.
-   * @returns (canEvaluateInLeft, canEvaluateInRight, haveToEvaluateInBoth)
+   * @return (canEvaluateInLeft, canEvaluateInRight, haveToEvaluateInBoth)
    */
   private def split(condition: Seq[Expression], left: LogicalPlan, right: LogicalPlan) = {
     val (leftEvaluateCondition, rest) =