sse2 intrinsic version of vp8_mbloop_filter_horizontal_edge()

First sse2 version of vp8_mbloop_filter_horizontal_edge().  For now,
intrinsics are being used until the bitstream is finalized.  This function
will be revisited later for further performance improvements.
For the test clip used, a 31+% decoder performance improvement
was seen.  This will vary depending on material.

Change-Id: I03ed3a7182478bdd1f094644ff3e0442625600e7