adding register_names to instruction protocol

lib/risc/instructions/getter.rb

@@ -29,6 +29,13 @@ module Risc
     end
     attr_accessor :array , :index , :register
 
+    # return an array of names of registers that is used by the instruction
+    def register_names
+      names = [array.symbol , register.symbol]
+      names << index.symbol if index.is_a?(RegisterValue)
+      names
+    end
+
     def to_s
       class_source "#{array}[#{index}] -> #{register}"
     end