target/arm: Implement MVE VHADD, VHSUB

Implement MVE VHADD and VHSUB insns, which perform an addition
or subtraction and then halve the result.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210617121628.20116-18-peter.maydell@linaro.org
4 files changed