Update variance sse2 functions

Update variance sse2 functions

Old vs New
Variance 64x64 time:   197 ms   143 ms
Variance 64x32 time:   200 ms   146 ms
Variance 32x64 time:   203 ms   140 ms
Variance 32x32 time:   214 ms   152 ms
Variance 32x16 time:   243 ms   153 ms
Variance 16x32 time:   234 ms   197 ms
Variance 16x16 time:   205 ms   205 ms
Variance 16x8  time:   228 ms   222 ms
Variance 8x16  time:   228 ms   232 ms
Variance 8x8   time:   282 ms   240 ms
Variance 8x4   time:   506 ms   341 ms
Variance 4x8   time:   518 ms   415 ms
Variance 4x4   time:   604 ms   628 ms

Observed vp9 encoder speed up when encoding a 720p video.

Change-Id: Iebb98f3b3d8adbc11a733a529d8427ce3d2a5314