Convert SimpleCall to new regs

Also fix bug in RegsiterValue/Slot with chain, where logic was dodgy and compiler not set

test/risc/test_register_slot.rb

@@ -72,4 +72,27 @@ module Risc
       assert_equal :message , inst.array.symbol
     end
   end
+  class TestRegisterSlot4 < MiniTest::Test
+    def setup
+      Parfait.boot!(Parfait.default_test_options)
+      @compiler = Risc.test_compiler
+      @r0 = RegisterValue.new(:message , :Message).set_compiler(@compiler)
+      @r0[:next_message][:return_address] << @r0
+    end
+    def test_instructions
+      assert_equal SlotToReg ,  @compiler.risc_instructions.next(1).class
+      assert_equal RegToSlot ,  @compiler.risc_instructions.next(2).class
+      assert_equal NilClass ,  @compiler.risc_instructions.next(3).class
+    end
+    def test_slot_to
+      slot = @compiler.risc_instructions.next(1)
+      assert_slot_to_reg slot , :message, 1, :"message.next_message"
+      assert slot.register.compiler
+    end
+    def test_reg_to
+      reg = @compiler.risc_instructions.next(2)
+      assert_reg_to_slot reg , :message, :"message.next_message" , 4
+      assert reg.register.compiler
+    end
+  end
 end