Implement sse2 and ssse3 versions for all sub_pixel_variance sizes.
Overall speedup around 5% (bus @ 1500kbps first 50 frames 4min10 ->
3min58). Specific changes to timings for each function compared to
original assembly-optimized versions (or just new version timings if
no previous assembly-optimized version was available):
sse2 4x4: 99 -> 82 cycles
sse2 4x8: 128 cycles
sse2 8x4: 121 cycles
sse2 8x8: 149 -> 129 cycles
sse2 8x16: 235 -> 245 cycles (?)
sse2 16x8: 269 -> 203 cycles
sse2 16x16: 441 -> 349 cycles
sse2 16x32: 641 cycles
sse2 32x16: 643 cycles
sse2 32x32: 1733 -> 1154 cycles
sse2 32x64: 2247 cycles
sse2 64x32: 2323 cycles
sse2 64x64: 6984 -> 4442 cycles
ssse3 4x4: 100 cycles (?)
ssse3 4x8: 103 cycles
ssse3 8x4: 71 cycles
ssse3 8x8: 147 cycles
ssse3 8x16: 158 cycles
ssse3 16x8: 188 -> 162 cycles
ssse3 16x16: 316 -> 273 cycles
ssse3 16x32: 535 cycles
ssse3 32x16: 564 cycles
ssse3 32x32: 973 cycles
ssse3 32x64: 1930 cycles
ssse3 64x32: 1922 cycles
ssse3 64x64: 3760 cycles
Change-Id: I81ff6fe51daf35a40d19785167004664d7e0c59d