target/s390x: Improve cc computation for SUBTRACT LOGICAL

target/s390x: Improve cc computation for SUBTRACT LOGICAL

The resulting cc is only dependent on the result and the carry-out.
Carry-out and borrow-out are inverses, so are trivially converted.
With tcg ops, it is easier to compute borrow-out than carry-out, so
save result and borrow-out rather than the inputs.

Borrow-out for 64-bit inputs is had via tcg_gen_sub2_i64 directly
into cc_src.  Borrow-out for 32-bit inputs is had via extraction
from a normal 64-bit sub (with zero-extended inputs).

Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20201214221356.68039-4-richard.henderson@linaro.org>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>