Upgrade vpx_lpf_{vertical,horizontal}_4 mmx to sse2

Followed the code style of other lpf fuctions.
These 2 functions put 2 rows of data in a single xmm register,
so they have similar but not identical filter operations,
and cannot share the same macros.

Change-Id: I3bab55a5d1a1232926ac8fd1f03251acc38302bc