CkArray: Reorder ckDestroy to avoid extraneous move

CkArray::deleteElt calls pop_back when removing from the local element
vector, so follow the same access pattern when destroying the elements
to avoid an unnecessary move in deleteElt.

Author: rbuch

src/ck-core/ckarray.C

@@ -1770,9 +1770,9 @@ void CkArray::ckDestroy()
   locMgr->setDuringDestruction(true);
 
   // ckDestroy deletes the CkMigratable, which also removes it from this vector
-  while (localElemVec.size())
+  while (!localElemVec.empty())
   {
-    localElemVec.front()->ckDestroy();
+    localElemVec.back()->ckDestroy();
   }
 
   locMgr->deleteManager(CkGroupID(thisProxy), this);