cheriot_instr_auipcc_cross impossibilities

[nwf]

@kliuMsft and I spent a while looking and eventually remembered that the cp_cd_pcc_repr_cases component of this cross product is describing whether the result is in bounds of PCC.

This implies that, for any PCC with exponent zero, any non-zero auipcc immediate will always produce a result in cd that is out of bounds (case2 of cp_cd_pcc_repr_cases) and so there needs to be some exclusion from the cross-product here.

[nwf]

Similar observations will hold for cp_cd_cs1_repr_cases / cp_cs1_exp / cp_cheri_imm20 for cheriot_instr_auicgp_cross. A subset will also hold for cheriot_instr_cincaddrimm_cross, I think?