Make opcode 00ff be called "dispatch-ff".

With this change, it's still implemented as an unused opcode, but
it's now ready for its new life!

Change-Id: Ic70d311704925067e47d87b657d133a792144e65
diff --git a/vm/compiler/Dataflow.c b/vm/compiler/Dataflow.c
index dad52bf..924ba62 100644
--- a/vm/compiler/Dataflow.c
+++ b/vm/compiler/Dataflow.c
@@ -795,7 +795,7 @@
     // FE OP_SPUT_OBJECT_VOLATILE
     DF_UA,
 
-    // FF OP_UNUSED_FF
+    // FF OP_DISPATCH_FF
     DF_NOP,
 
     // Beginning of extended MIR opcodes