Port ARGBToAR30Row_AVX2 to ARGBToAR30Row_SSE2 using same instructions
but xmm registers and doing half as many pixels per loop.

Bug: libyuv:751
Test: LibYUVConvertTest.ARGBToAR30_Opt
Change-Id: Id644e54639133d1caf28ea3cd11ff6ab6891a673
Reviewed-on: https://chromium-review.googlesource.com/817918
Commit-Queue: Frank Barchard <fbarchard@chromium.org>
Reviewed-by: richard winterton <rrwinterton@gmail.com>