Rename h264_weight_sse2.asm to h264_weight.asm; add 16x8/8x16/8x4 non-square

biweight code to sse2/ssse3; add sse2 weight code; and use that same code to
create mmx2 functions also, so that the inline asm in h264dsp_mmx.c can be
removed. OK'ed by Jason on IRC.

Originally committed as revision 25019 to svn://svn.ffmpeg.org/ffmpeg/trunk