split create_binary into two phases

Which gives instructions a chance to check everything
and in Arms case check the constant loads/ instruction adding
So that during assembly no more change happens (and we don't have to reassemble)

lib/arm/instructions/instruction.rb

@@ -1,4 +1,6 @@
 require "util/list"
+require "util/dev_null"
+
 module Arm
   # Arm instruction base class
   # Mostly linked list functionality that all instructions have
@@ -21,6 +23,13 @@ module Arm
       ret
     end
 
+    # precheck that everything is ok, before asembly
+    # in arm, we use the oppertunity to assemble to dev_null, so any
+    # additions are done _before_ assemnly
+    def precheck
+      assemble(Util::DevNull.new)
+    end
+
     def insert(instruction)
       ret = super
       Risc::Position.get(self).trigger_inserted if Risc::Position.set?(self)