target/arm: Implement MVE vector shift right by immediate insns

Implement the MVE vector shift right by immediate insns VSHRI and
VRSHRI.  As with Neon, we implement these by using helper functions
which perform left shifts but allow negative shift counts to indicate
right shifts.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210628135835.6690-9-peter.maydell@linaro.org