.part() is converted to Verilog that can evaluate to 'x

[whitequark]

.part() as implemented corresponds to the semantics of Yosys $shiftx cell. It should use the $shift cell. See https://github.com/YosysHQ/yosys/pull/744 for an example of how to emit Verilog for that cell.

[whitequark]

Triage: fixed in nMigen.