target-s390: Tidy s->op_cc handling

There's no need to force computation of the true cc_op when taking an
exception or single stepping.  In either case we'll enter the next TB
with s->cc_op = DYNAMIC and recompute anyway.  Just make sure that
s->cc_op is stored back to env->cc_op as needed.

Delete some dead functions, avoid allocating unused TCG temps, drop
the old s->is_jmp setting.

Signed-off-by: Richard Henderson <rth@twiddle.net>