fix slot_to_reg to allow register indexes

we mostly use pre-calculated indexes, ie integers
but registers are allowed (in arm/risc), so we try to check the registers type at least is right.
The return is really a machine word, but we call it Object (yes, more work that way)

test/risc/instructions/test_slot_to_reg.rb

@@ -14,5 +14,13 @@ module Risc
     def test_slot_reg
       assert_equal :"message.type" , slot.register.symbol
     end
+    def test_reg_as_index
+      reg = RegisterValue.new(:name , :Integer)
+      Risc.slot_to_reg("source" , Risc.message_named_reg , reg)
+    end
+    def test_reg_as_index_fail
+      reg = Risc.message_named_reg
+      assert_raises {Risc.slot_to_reg("source" , reg , reg)}
+    end
   end
 end