adding register_names to instruction protocol

lib/risc/instructions/load_data.rb

@@ -17,6 +17,11 @@ module Risc
     end
     attr_accessor :register , :constant
 
+    # return an array of names of registers that is used by the instruction
+    def register_names
+      [register.symbol]
+    end
+
     def to_s
       class_source "#{register} <- #{constant}"
     end