Considering a horizontal edge, if mask_16x16 is 1 for an even-
indexed 8x8 block, then mask_16x16 is 1 for next 8x8 block in
same row. Similiar to a verticle edge, if mask_16x16 is 1 for
an even-rowed 8x8 block, then mask_16x16 is 1 for the 8x8 block
right below it in next raw. Based on that, the mask_16x16 checking
can be simplified to save cycles. The corresponding 8-pixel
vp9_mb_lpf_horizontal_edge code can also be removed.

Change-Id: Ic3fe7a5674322239208cbe2731dc3216ce2084f3