target/arm: Simplify do_long_3d and do_2scalar_long

In both cases, we can sink the write-back and perform
the accumulate into the normal destination temps.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201030022618.785675-11-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>