[BUGFIX release] improve performance of guidFor when reading an exist…

…ing guid

* This made OrderedSet#add 2x -> 3x faster (helps our ember-data friends)
* This appears to have made OrderedSet#add faster then V8 native Set#add (for now)

(cherry picked from commit a19e1f9)

packages/ember-metal/lib/utils.js

@@ -215,6 +215,9 @@ export function generateGuid(obj, prefix) {
   @return {String} the unique guid for this instance.
 */
 export function guidFor(obj) {
+  if (obj && obj[GUID_KEY]) {
+    return obj[GUID_KEY];
+  }
 
   // special cases where we don't want to add a key to object
   if (obj === undefined) {
@@ -252,10 +255,6 @@ export function guidFor(obj) {
       return obj ? '(true)' : '(false)';
 
     default:
-      if (obj[GUID_KEY]) {
-        return obj[GUID_KEY];
-      }
-
       if (obj === Object) {
         return '(Object)';
       }