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.

nmigen/back/rtlil.py

@@ -772,6 +772,7 @@ def convert_fragment(builder, fragment, hierarchy):
 
             sub_ports = OrderedDict()
             for port, value in sub_port_map.items():
+                if not isinstance(subfragment, ir.Instance):
                     for signal in value._rhs_signals():
                         compiler_state.resolve_curr(signal, prefix=sub_name)
                 sub_ports[port] = rhs_compiler(value)