[SPARK-4454] Revert getOrElse() cleanup in DAGScheduler.getCacheLocs()

This method is performance-sensitive and this change wasn't necessary.

core/src/main/scala/org/apache/spark/scheduler/DAGScheduler.scala

@@ -190,13 +190,15 @@ class DAGScheduler(
   }
 
   private def getCacheLocs(rdd: RDD[_]): Array[Seq[TaskLocation]] = cacheLocs.synchronized {
-    cacheLocs.getOrElseUpdate(rdd.id, {
+    // Note: this doesn't use `getOrElse()` because this method is called O(num tasks) times
+    if (!cacheLocs.contains(rdd.id)) {
       val blockIds = rdd.partitions.indices.map(index => RDDBlockId(rdd.id, index)).toArray[BlockId]
       val locs = BlockManager.blockIdsToBlockManagers(blockIds, env, blockManagerMaster)
-      blockIds.map { id =>
+      cacheLocs(rdd.id) = blockIds.map { id =>
         locs.getOrElse(id, Nil).map(bm => TaskLocation(bm.host, bm.executorId))
       }
-    })
+    }
+    cacheLocs(rdd.id)
   }
 
   private def clearCacheLocs(): Unit = cacheLocs.synchronized {