[ot-shape] Micro-optimize ::equal

For two integers, it's faster to compare them directly.

src/hb-ot-shape.hh

@@ -51,7 +51,8 @@ struct hb_ot_shape_plan_key_t
 
   bool equal (const hb_ot_shape_plan_key_t *other)
   {
-    return 0 == hb_memcmp (this, other, sizeof (*this));
+    return variations_index[0] == other->variations_index[0] &&
+	   variations_index[1] == other->variations_index[1];
   }
 };