Do vertical loopfiltering in parallel

This patch followed "Add filter_selectively_vert_row2 to enable
parallel loopfiltering" commit, and added x86 SSE2 optimization
to do 16-pixel filtering in parallel. For other optimizations
(neon and dspr2), current 16-pixel functions were done by calling
8-pixel functions twice, and real 16-pixel functions could be added
later.

Decoder speedup:
tulip clip:     2% speed gain;
old_town_cross: 1.2% speed gain;
bus:            2% speed gain.

Change-Id: I4818a0c72f84b34f5fe678e496cf4a10238574b7