comp_mask_pred:process each width separately
comp_mask_pred:process each width separately

There are 3 valid input width of aom_comp_mask_pred_ssse3.
Process each width(8,16,32) separately achieves
1.2x~1.5x speed up compare to origin ssse3 version.

Change-Id: Ida3699e2e6ca98d1f9c7662d48806b299af26f10

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom