chkPS, used to be only for words that pushed back to PS, but I've
recently removed underflow checks from next and I forgot to add
missing chkPS, calls that go with this change.
it seems I left my asm argument harmonization half done here.
Instructions list at B208 doesn't correspond to many actual
mnemonics. This mnemonic here was the worst offender.
Initially, I used the same letters as those used in the z80 ref
docs, but it makes the different assemblers harder to use than they
should. Having consistent "argtype" rules across assemblers should
help.