This version of the H010ToAR30 provides a 3 step conversion
Convert16To8Row_AVX2
H420ToARGB_AVX2
ARGBToAR30_AVX2

Low level function added to convert 16 bit to 8 bit using multiply
to adjust 10 bit or other bit depths and then save the upper 16 bits.

Bug: libyuv:751
Test: LibYUVPlanarTest.Convert16To8Row_Opt unittest added
Change-Id: I9cc576fda8afa1003cb961d03e0e656e0b478f03
Reviewed-on: https://chromium-review.googlesource.com/783554
Commit-Queue: Frank Barchard <fbarchard@chromium.org>
Reviewed-by: richard winterton <rrwinterton@gmail.com>