All `proc_*` passes now use the same module and process for loops, using `design->all_selected_modules()` and `mod->selected_processes()` respectively.
This simplifies the code, and makes the couple `proc_*` passes that were ignoring boxed modules stop doing that (which seems to have been erroneous rather than intentional).
See the test case. PROC_ROM will consider this for evaluation, even
though -- without any actions -- lhs is empty (but still "uniform").
A zero-width memory is constructed, which later fails check with:
ERROR: Assert `width != 0' failed in kernel/mem.cc:518.
Ensure we don't proceed if there's nothing to encode.
