GitLab

C vs SSE2 speed gains:
_4x4 : ~1.81x

C vs SSSE3 speed gains:
_8x8 : ~1.96x
_16x16 : ~1.88x
_32x32 : ~2.02x

BUG=webm:1411

Change-Id: Iefaf8b39afbbfe34c1ad1d21e3a003b20f1f61e0