target/s390x: Improve SUB LOGICAL WITH BORROW

Now that SUB LOGICAL outputs borrow, we can use that as input directly.
It also means we can re-use CC_OP_SUBU and produce an output borrow
directly from SUB LOGICAL WITH BORROW.

Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20201214221356.68039-5-richard.henderson@linaro.org>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>