target/arm: Improve do_prewiden_3d

We can use proper widening loads to extend 32-bit inputs,
and skip the "widenfn" step.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201030022618.785675-12-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>