Improve SSE2 loopfilter functions

Restructured and rewrote SSE2 loopfilter functions. Combined u and
v into one function to take advantage of SSE2 128-bit registers.
Tests on test clips showed a 4% decoder performance improvement on
Linux desktop.

Change-Id: Iccc6669f09e17f2224da715f7547d6f93b0a4987
4 files changed