def emit_builtin_reg_class(node)
      arg = node.args[0]
      Util.assert!{ arg.ty.kind == Kind::PTR }
      ty = arg.ty.ptr
      if ty.kind == Kind::STRUCT
        emit("mov $2, #eax")
      elsif Type.is_flotype(ty)
        emit("mov $1, #eax")
      else
        emit("mov $0, #eax")
      end
    end
def emit_builtin_reg_class(node)
      arg = node.args[0]
      Util.assert!{ arg.ty.kind == Kind::PTR }
      ty = arg.ty.ptr
      if ty.kind == Kind::STRUCT
        emit("mov $2, #eax")
      elsif Type.is_flotype(ty)
        emit("mov $1, #eax")
      else
        emit("mov $0, #eax")
      end
    end
Set the register class for parameter passing to RAX.
 0 is INTEGER, 1 is SSE, 2 is MEMORY.

 @param [Node] node