back.rtlil: don't name-prefix signals connected to instance ports.

back.rtlil: don't name-prefix signals connected to instance ports.

This gives particularly pathological results on IO buffers, like:
  connect \D_OUT_0 \user_led_0_user_led_0__o

Since subfragment signals are name-prefixed because this works well
for signals propagated upwards across hierarchy, this is never
desirable for instances.