BUG: non-fixed-length ISAs are unsupported for now

[Jose E. Marchesi]

>> I have tried it. Result is the same: ifields are just added (part of
>> generated z80-opc.c):
>
> I'm unfortunately unable to actually run cgen here (apparent
> incompatibility with "new" fedora32-level guile), so am only guessing
> that you could try setting the cpu base-insn-size to 16 or even 32:
> enough bits to tell the instructions apart.

That looks like the right approach.

Currently CGEN does not support having opcodes (constant fields) past
the base part of the insn, which by the way is limited to a maximum of
32-bit.