Added filter_selectively_vert_row2 to be ready for parallel
loopfiltering in vertical direction. This change did 2-row
filtering at a time. If 2 vertically adjacent 8x8 blocks do same
type of filtering, we can do 16-pixel filtering in parallel.

Next, we need to provide 16-pixel loopfiltering functions in c
and optimized versions for codec speedup.

Change-Id: Idf97bbdd70566e55bd30e1fd25cb8544e33291be