Since the packed cell doesn't actually support wide ports yet, we just
auto-narrow them on emit. The future packed cell will add
RD_WIDE_CONTINUATION and WR_WIDE_CONTINUATION parameters so the
transform will be trivially reversible for proper serialization.
Such ports cannot actually be created or used yet, this just adds the
necessary plumbing in the helper. Subsequent commits will gradually
add wide port support to various yosys passes.
This is going to be used to store arbitrary priority masks in the
future. Right now, it is not supported by our cell library, so the
priority_mask is computed from port order on helper construction,
and discarded when emitted. However, this allows us to already convert
helper-using passes to the new model.
