target/arm: Implement MVE VMLADAV and VMLSLDAV

target/arm: Implement MVE VMLADAV and VMLSLDAV

Implement the MVE VMLADAV and VMLSLDAV insns.  Like the VMLALDAV and
VMLSLDAV insns already implemented, these accumulate multiplied
vector elements; but they accumulate a 32-bit result rather than a
64-bit one.

Note that these encodings overlap with what would be RdaHi=0b111 for
VMLALDAV, VMLSLDAV, VRMLALDAVH and VRMLSLDAVH.

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