[SPARK-5419][Mllib] Fix the logic in Vectors.sqdist

The current implementation in Vectors.sqdist is not efficient because of allocating temp arrays. There is also a bug in the code `v1.indices.length / v1.size < 0.5`. This pr fixes the bug and refactors sqdist without allocating new arrays.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #4217 from viirya/fix_sqdist and squashes the following commits:

e8b0b3d [Liang-Chi Hsieh] For review comments.
314c424 [Liang-Chi Hsieh] Fix sqdist bug.

mllib/src/main/scala/org/apache/spark/mllib/linalg/Vectors.scala

@@ -371,18 +371,23 @@ object Vectors {
           squaredDistance += score * score
         }
 
-      case (v1: SparseVector, v2: DenseVector) if v1.indices.length / v1.size < 0.5 =>
+      case (v1: SparseVector, v2: DenseVector) =>
         squaredDistance = sqdist(v1, v2)
 
-      case (v1: DenseVector, v2: SparseVector) if v2.indices.length / v2.size < 0.5 =>
+      case (v1: DenseVector, v2: SparseVector) =>
         squaredDistance = sqdist(v2, v1)
 
-      // When a SparseVector is approximately dense, we treat it as a DenseVector
-      case (v1, v2) =>
-        squaredDistance = v1.toArray.zip(v2.toArray).foldLeft(0.0){ (distance, elems) =>
-          val score = elems._1 - elems._2
-          distance + score * score
+      case (DenseVector(vv1), DenseVector(vv2)) =>
+        var kv = 0
+        val sz = vv1.size
+        while (kv < sz) {
+          val score = vv1(kv) - vv2(kv)
+          squaredDistance += score * score
+          kv += 1
         }
+      case _ =>
+        throw new IllegalArgumentException("Do not support vector type " + v1.getClass +
+          " and " + v2.getClass)
     }
     squaredDistance
   }