Nesting Parts with a variable offset throws an AssertionError

[jfng]

Repro:

from nmigen import *
from nmigen.back import rtlil

m = Module()

data = Signal(4)
sel  = Signal()

part_0 = data.word_select(sel, width=2)
part_1 = part_0.word_select(0, width=1)

m.d.comb += part_1.eq(1)

print(rtlil.convert(m, ports=[data, sel]))

Output:

Traceback (most recent call last):
  File "/tmp/repro.py", line 14, in <module>
    print(rtlil.convert(m, ports=[data, sel]))
  File "/home/jf/src/nmigen/nmigen/back/rtlil.py", line 1054, in convert
    il_text, name_map = convert_fragment(fragment, name)
  File "/home/jf/src/nmigen/nmigen/back/rtlil.py", line 1048, in convert_fragment
    _convert_fragment(builder, fragment, name_map, hierarchy=(name,))
  File "/home/jf/src/nmigen/nmigen/back/rtlil.py", line 942, in _convert_fragment
    stmt_compiler(group_stmts)
  File "/home/jf/src/nmigen/nmigen/hdl/xfrm.py", line 232, in __call__
    return self.on_statement(stmt)
  File "/home/jf/src/nmigen/nmigen/back/rtlil.py", line 783, in on_statement
    super().on_statement(stmt)
  File "/home/jf/src/nmigen/nmigen/hdl/xfrm.py", line 220, in on_statement
    new_stmt = self.on_statements(stmt)
  File "/home/jf/src/nmigen/nmigen/back/rtlil.py", line 801, in on_statements
    self.on_statement(stmt)
  File "/home/jf/src/nmigen/nmigen/back/rtlil.py", line 783, in on_statement
    super().on_statement(stmt)
  File "/home/jf/src/nmigen/nmigen/hdl/xfrm.py", line 209, in on_statement
    new_stmt = self.on_Assign(stmt)
  File "/home/jf/src/nmigen/nmigen/back/rtlil.py", line 731, in on_Assign
    self._case.assign(self.lhs_compiler(stmt.lhs), rhs_sigspec)
  File "/home/jf/src/nmigen/nmigen/hdl/xfrm.py", line 127, in __call__
    return self.on_value(value)
  File "/home/jf/src/nmigen/nmigen/hdl/xfrm.py", line 104, in on_value
    new_value = self.on_Slice(value)
  File "/home/jf/src/nmigen/nmigen/back/rtlil.py", line 398, in on_Slice
    sigspec = self._prepare_value_for_Slice(value.value)
  File "/home/jf/src/nmigen/nmigen/back/rtlil.py", line 661, in _prepare_value_for_Slice
    assert isinstance(value, (ast.Signal, ast.Slice, ast.Cat))
AssertionError

This error does not occur if sel is replaced by a constant offset in the above repro.