AR30ToARGB will vectorize if the output is masked
together as an int instead of 4 byte stores.
Performance is 2x faster
Was AR30ToARGB_Opt (1585 ms)
Now AR30ToARGB_Opt (746 ms)

Bug: libyuv:777
Test:LibYUVConvertTest.AR30ToARGB_Opt
Change-Id: Idd47ae599d5d125207bb53e618d6d7e784d4a37c
Reviewed-on: https://chromium-review.googlesource.com/923169Reviewed-by: Miguel Casas <mcasas@chromium.org>
Commit-Queue: Frank Barchard <fbarchard@chromium.org>