An SSSE3 version already exists, and an AVX2 version is available for
Visual C.  This ports the function to AVX2 completing the AVX2 ports of
all YUV to RGB functions for AVX2 on gcc.

TBR=harryjin@google.com
BUG=libyuv:555

Review URL: https://codereview.chromium.org/1687253002 .