There's a mismatch between what `kernel/mem.cc` emits for memories
with no read ports and what the internal RTLIL check expects.
The point of dispute it whether some of the parameters relating to read
ports have a zero-width value in this case. The `mem.cc` code says no,
the internal checker says yes.
Surveying the other `$mem_v2` parameters, and internal cell parameters
in general, I am inclined to side with the `mem.cc` code.
This breaks RTLIL compatibility but for an obscure edge case.
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