target/arm: Implement widening/narrowing MVE VLDR/VSTR insns

target/arm: Implement widening/narrowing MVE VLDR/VSTR insns

Implement the variants of MVE VLDR (encodings T1, T2) which perform
"widening" loads where bytes or halfwords are loaded from memory and
zero or sign-extended into halfword or word length vector elements,
and the narrowing MVE VSTR (encodings T1, T2) where bytes or
halfwords are stored from halfword or word elements.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210617121628.20116-3-peter.maydell@linaro.org