sse2 v. C speedup:
4x4   ~8.0x
8x8   ~8.2x
16x16 ~6.5x
32x32 ~3.8x
Blocksize:
4x4, 4x8, 8x4, 8x8, 8x16, 16x8, 16x16, 16x32, 32x16, 32x32
Square blocksize code is from libvpx:
"30d9a1916 vpxdsp: [x86] add highbd_h_predictor functions",
Credit goes to Scott LaVarnway. Speed tests do not support
rectangular blocksize yet.

Change-Id: I9a1f24aecab8de94f8ea59ec8748fe3537d721ae