target/s390x: Improve cc computation for ADD LOGICAL

The resulting cc is only dependent on the result and the
carry-out.  So save those things rather than the inputs.

Carry-out for 64-bit inputs is had via tcg_gen_add2_i64 directly
into cc_src.  Carry-out for 32-bit inputs is had via extraction
from a normal 64-bit add (with zero-extended inputs).

Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20201214221356.68039-2-richard.henderson@linaro.org>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>