Computing carry is trivial for some inputs.  By avoiding an
external function call, we generate near-optimal code for
the common cases of add+addx (double-word arithmetic) and
cmp+addx (a setcc pattern).
Signed-off-by: NRichard Henderson <rth@twiddle.net>
Acked-by: NArtyom Tarasenko <atar4qemu@gmail.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>