Implement the MVE shift-vector-left-by-immediate insns VSHL, VQSHL
and VQSHLU.

The size-and-immediate encoding here is the same as Neon, and we
handle it the same way neon-dp.decode does.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210628135835.6690-8-peter.maydell@linaro.org