[SPARK-21491][GRAPHX] Enhance GraphX performance: breakOut instead of .toMap

## What changes were proposed in this pull request?

`Traversable.toMap` changed to 'collections.breakOut', that eliminates intermediate tuple collection creation, see [Stack Overflow article](https://stackoverflow.com/questions/1715681/scala-2-8-breakout).

## How was this patch tested?
Unit tests run.
No performance tests performed yet.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: iurii.ant <sereneant@gmail.com>

Closes #18693 from SereneAnt/performance_toMap-breakOut.

graphx/src/main/scala/org/apache/spark/graphx/lib/LabelPropagation.scala

@@ -55,7 +55,7 @@ object LabelPropagation {
         val count1Val = count1.getOrElse(i, 0L)
         val count2Val = count2.getOrElse(i, 0L)
         i -> (count1Val + count2Val)
-      }.toMap
+      }(collection.breakOut) // more efficient alternative to [[collection.Traversable.toMap]]
     }
     def vertexProgram(vid: VertexId, attr: Long, message: Map[VertexId, Long]): VertexId = {
       if (message.isEmpty) attr else message.maxBy(_._2)._1

graphx/src/main/scala/org/apache/spark/graphx/lib/ShortestPaths.scala

@@ -33,10 +33,11 @@ object ShortestPaths {
 
   private def incrementMap(spmap: SPMap): SPMap = spmap.map { case (v, d) => v -> (d + 1) }
 
-  private def addMaps(spmap1: SPMap, spmap2: SPMap): SPMap =
+  private def addMaps(spmap1: SPMap, spmap2: SPMap): SPMap = {
     (spmap1.keySet ++ spmap2.keySet).map {
       k => k -> math.min(spmap1.getOrElse(k, Int.MaxValue), spmap2.getOrElse(k, Int.MaxValue))
-    }.toMap
+    }(collection.breakOut) // more efficient alternative to [[collection.Traversable.toMap]]
+  }
 
   /**
    * Computes shortest paths to the given set of landmark vertices.