target/arm: Implement MVE VCADD

target/arm: Implement MVE VCADD

Implement the MVE VCADD insn, which performs a complex add with
rotate.  Note that the size=0b11 encoding is VSBC.

The architecture grants some leeway for the "destination and Vm
source overlap" case for the size MO_32 case, but we choose not to
make use of it, instead always calculating all 16 bytes worth of
results before setting the destination register.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210617121628.20116-42-peter.maydell@linaro.org