def use_reg( type , extra = {} )
      raise "Not type #{type.inspect}" unless type.is_a?(Symbol) or type.is_a?(Parfait::Type)
      if @regs.empty?
        reg = Risc.tmp_reg(type , extra)
      else
        reg = @regs.last.next_reg_use(type , extra)
      end
      @regs << reg
      return reg
    end
def use_reg( type , extra = {} )
      raise "Not type #{type.inspect}" unless type.is_a?(Symbol) or type.is_a?(Parfait::Type)
      if @regs.empty?
        reg = Risc.tmp_reg(type , extra)
      else
        reg = @regs.last.next_reg_use(type , extra)
      end
      @regs << reg
      return reg
    end
require a (temporary) register. code must give this back with release_reg
 Second extra parameter may give extra info about the value, see RegisterValue