NV12ToRGB565Row for Intel is implemented as a 2 step conversion:
NV12ToARGBRow_SSSE3 and ARGBToRGB565Row_SSE2

NV12ToARGBRow has an AVX2 version, so this CL implements
NV12ToRGB565Row_AVX2 with call to NV12ToARGBRow_AVX2 and
ARGBToRGB565Row_SSE2.

R=harryjin@google.com
BUG=libyuv:554

Review URL: https://codereview.chromium.org/1687953002 .