more ripples from removing index constants

lib/virtual/passes/enter_implementation.rb

@@ -5,9 +5,9 @@ module Virtual
       block.codes.dup.each do |code|
         next unless code.is_a? Virtual::MethodEnter
         new_codes = []
-        # save return register and create a new frame
-        # lr is link register, ie where arm stores the return address when call is issued
-        new_codes << Register::SaveReturn.new( Register::RegisterReference.message_reg , Virtual::RETURN_INDEX )
+        # save return register to the message at instance return_address
+        new_codes << Register.save_return(:message , :return_address)
+        # and create a new frame if needed
         unless code.method.locals.empty? and code.method.tmps.empty?
           new_codes << Virtual::NewFrame.new
         end