target/arm: Split decode of VSDOT and VUDOT

Now that we have a common helper, sharing decode does not
save much.  Also, this will solve an upcoming naming problem.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210525010358.152808-89-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>