Improved mb_lpf_horizontal_edge_w_sse2_8

This patch is a reformatted version of optimizations done by
engineers at Intel (Erik/Tamar) who have been providing
performance feedback for VP9.  For the test clips used (720p, 1080p),
up to 1.2% performance improvement was seen.

Change-Id: Ic1a7149098740079d5453b564da6fbfdd0b2f3d2