Fabric: Use ShadowNode::sameFamily instead of getTag() to check for same family

Summary:
We used to use `getTag` to check that some two nodes are clones of each other, not we have a dedicated method for that that exactly ensures that without reling  on a sideeffect (wich tag equality is).
That is just much less error-prone.

Changelog: [Internal] Fabric-specific internal change.

Reviewed By: mdvacca

Differential Revision: D18231006

fbshipit-source-id: 6b247ed0eaded1fed8fd7fa820e80cd58602110c

ReactCommon/fabric/mounting/ShadowTree.cpp

@@ -59,7 +59,7 @@ static void updateMountedFlag(
       continue;
     }
 
-    if (oldChild->getTag() != newChild->getTag()) {
+    if (!ShadowNode::sameFamily(*oldChild, *newChild)) {
       // Totally different nodes, updating is impossible.
       break;
     }