Bug 744052 - Restructure HashTable::checkOverflow for easier reading; r=luke

When I added this function, I just copied the existing logical structure.  It
makes more sense here to check overflowed and return early, since we can do this
now.

js/public/HashTable.h

@@ -619,7 +619,9 @@ class HashTable : private AllocPolicy
 
     bool checkOverloaded()
     {
-        if (overloaded()) {
+        if (!overloaded())
+            return false;
+
         /* Compress if a quarter or more of all entries are removed. */
         int deltaLog2;
         if (removedCount >= (capacity() >> 2)) {
@@ -633,9 +635,6 @@ class HashTable : private AllocPolicy
         return changeTableSize(deltaLog2);
     }
 
-        return false;
-    }
-
     void remove(Entry &e)
     {
         METER(stats.removes++);