1) For encoder speed, overall ~1% faster with no impact on coding performance.
2) aom_comp_mask_pred_ssse3 is 3.5x - 6x faster than aom_comp_mask_pred_c
3) aom_comp_mask_upsampled_pred_ssse3 1.5x - 3x faster than
aom_comp_mask_upsampled_pred_c, for special case where subpel_x ==
subpel_y == 0, optimized version achieves 4x - 7x speedup

Unittest for both functions have been added.

Change-Id: Ib498317975e0dbd9cdcf61be327b640dfac9a7e5