Use word-writing instead of dword-writing (with two cached but otherwise

Use word-writing instead of dword-writing (with two cached but otherwise
unchanged bytes) in the horizontal simple loopfilter. This makes the filter
quite a bit faster in itself (~30 cycles less on Core1), probably mostly
because we don't need a complex 4x4 transpose, but only a simple byte
interleave. Also allows using pextrw on SSE4, which speeds up even more
(e.g. 25% faster on Core i7).

Originally committed as revision 24638 to svn://svn.ffmpeg.org/ffmpeg/trunk