hdl.ast, back.rtlil: add source locations to anonymous wires.

This might help with propagation of locations through optimizer
passes, since not all of them take care to preserve cells at all,
but usually wires stay intact when possible.

Also fixes incorrect source location on value.part().

nmigen/hdl/ast.py

@@ -163,7 +163,7 @@ class Value(metaclass=ABCMeta):
         Part, out
             Selected part of the ``Value``
         """
-        return Part(self, offset, width)
+        return Part(self, offset, width, src_loc_at=1)
 
     def eq(self, value):
         """Assignment.