target/arm: Pass TCGMemOpIdx to sve memory helpers

There is quite a lot of code required to compute cpu_mem_index,
or even put together the full TCGMemOpIdx.  This can easily be
done at translation time.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20181005175350.30752-16-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>