target/arm: Implement MVE narrowing moves

target/arm: Implement MVE narrowing moves

Implement the MVE narrowing move insns VMOVN, VQMOVN and VQMOVUN.
These take a double-width input, narrow it (possibly saturating) and
store the result to either the top or bottom half of the output
element.

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