[SPARK-16331][SQL] Reduce code generation time

## What changes were proposed in this pull request?
During the code generation, a `LocalRelation` often has a huge `Vector` object as `data`. In the simple example below, a `LocalRelation` has a Vector with 1000000 elements of `UnsafeRow`.

```
val numRows = 1000000
val ds = (1 to numRows).toDS().persist()
benchmark.addCase("filter+reduce") { iter =>
  ds.filter(a => (a & 1) == 0).reduce(_ + _)
}
```

At `TreeNode.transformChildren`, all elements of the vector is unnecessarily iterated to check whether any children exist in the vector since `Vector` is Traversable. This part significantly increases code generation time.

This patch avoids this overhead by checking the number of children before iterating all elements; `LocalRelation` does not have children since it extends `LeafNode`.

The performance of the above example
```
without this patch
Java HotSpot(TM) 64-Bit Server VM 1.8.0_91-b14 on Mac OS X 10.11.5
Intel(R) Core(TM) i5-5257U CPU  2.70GHz
compilationTime:                         Best/Avg Time(ms)    Rate(M/s)   Per Row(ns)   Relative
------------------------------------------------------------------------------------------------
filter+reduce                                 4426 / 4533          0.2        4426.0       1.0X

with this patch
compilationTime:                         Best/Avg Time(ms)    Rate(M/s)   Per Row(ns)   Relative
------------------------------------------------------------------------------------------------
filter+reduce                                 3117 / 3391          0.3        3116.6       1.0X
```

## How was this patch tested?

using existing unit tests

Author: Hiroshi Inoue <inouehrs@jp.ibm.com>

Closes #14000 from inouehrs/compilation-time-reduction.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala

@@ -315,25 +315,9 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
   protected def transformChildren(
       rule: PartialFunction[BaseType, BaseType],
       nextOperation: (BaseType, PartialFunction[BaseType, BaseType]) => BaseType): BaseType = {
-    var changed = false
-    val newArgs = mapProductIterator {
-      case arg: TreeNode[_] if containsChild(arg) =>
-        val newChild = nextOperation(arg.asInstanceOf[BaseType], rule)
-        if (!(newChild fastEquals arg)) {
-          changed = true
-          newChild
-        } else {
-          arg
-        }
-      case Some(arg: TreeNode[_]) if containsChild(arg) =>
-        val newChild = nextOperation(arg.asInstanceOf[BaseType], rule)
-        if (!(newChild fastEquals arg)) {
-          changed = true
-          Some(newChild)
-        } else {
-          Some(arg)
-        }
-      case m: Map[_, _] => m.mapValues {
+    if (children.nonEmpty) {
+      var changed = false
+      val newArgs = mapProductIterator {
         case arg: TreeNode[_] if containsChild(arg) =>
           val newChild = nextOperation(arg.asInstanceOf[BaseType], rule)
           if (!(newChild fastEquals arg)) {
@@ -342,33 +326,53 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
           } else {
             arg
           }
-        case other => other
-      }.view.force // `mapValues` is lazy and we need to force it to materialize
-      case d: DataType => d // Avoid unpacking Structs
-      case args: Traversable[_] => args.map {
-        case arg: TreeNode[_] if containsChild(arg) =>
+        case Some(arg: TreeNode[_]) if containsChild(arg) =>
           val newChild = nextOperation(arg.asInstanceOf[BaseType], rule)
           if (!(newChild fastEquals arg)) {
             changed = true
-            newChild
+            Some(newChild)
           } else {
-            arg
+            Some(arg)
           }
-        case tuple @ (arg1: TreeNode[_], arg2: TreeNode[_]) =>
-          val newChild1 = nextOperation(arg1.asInstanceOf[BaseType], rule)
-          val newChild2 = nextOperation(arg2.asInstanceOf[BaseType], rule)
-          if (!(newChild1 fastEquals arg1) || !(newChild2 fastEquals arg2)) {
-            changed = true
-            (newChild1, newChild2)
-          } else {
-            tuple
-          }
-        case other => other
+        case m: Map[_, _] => m.mapValues {
+          case arg: TreeNode[_] if containsChild(arg) =>
+            val newChild = nextOperation(arg.asInstanceOf[BaseType], rule)
+            if (!(newChild fastEquals arg)) {
+              changed = true
+              newChild
+            } else {
+              arg
+            }
+          case other => other
+        }.view.force // `mapValues` is lazy and we need to force it to materialize
+        case d: DataType => d // Avoid unpacking Structs
+        case args: Traversable[_] => args.map {
+          case arg: TreeNode[_] if containsChild(arg) =>
+            val newChild = nextOperation(arg.asInstanceOf[BaseType], rule)
+            if (!(newChild fastEquals arg)) {
+              changed = true
+              newChild
+            } else {
+              arg
+            }
+          case tuple@(arg1: TreeNode[_], arg2: TreeNode[_]) =>
+            val newChild1 = nextOperation(arg1.asInstanceOf[BaseType], rule)
+            val newChild2 = nextOperation(arg2.asInstanceOf[BaseType], rule)
+            if (!(newChild1 fastEquals arg1) || !(newChild2 fastEquals arg2)) {
+              changed = true
+              (newChild1, newChild2)
+            } else {
+              tuple
+            }
+          case other => other
+        }
+        case nonChild: AnyRef => nonChild
+        case null => null
       }
-      case nonChild: AnyRef => nonChild
-      case null => null
+      if (changed) makeCopy(newArgs) else this
+    } else {
+      this
     }
-    if (changed) makeCopy(newArgs) else this
   }
 
   /**