Change copy-on-write protocol to always return valid pointers

Streamline the array access methods by always returning an array
pointer instead of a new pointer or null.  Callsites compare
(new != old) to detect escalation, rather than (new != null).

hphp/runtime/base/array/array_data.cpp

@@ -223,7 +223,7 @@ ArrayData *ArrayData::pop(Variant &value) {
     return remove(getKey(pos), getCount() > 1);
   }
   value = uninit_null();
-  return nullptr;
+  return this;
 }
 
 ArrayData *ArrayData::dequeue(Variant &value) {
@@ -233,16 +233,11 @@ ArrayData *ArrayData::dequeue(Variant &value) {
     ArrayData *ret = remove(getKey(pos), getCount() > 1);
 
     // In PHP, array_shift() will cause all numerically key-ed values re-keyed
-    if (ret) {
-      ret->renumber();
-    } else {
-      renumber();
-    }
-
+    ret->renumber();
     return ret;
   }
   value = uninit_null();
-  return nullptr;
+  return this;
 }
 
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