1. Add sse4_1 version of
av1_build_compound_diffwtd_mask
2. The unit test shows it is 4.2x~ 12.23x times
faster then C version
3. For encoder, about 0.6% faster shows by encoding
10 frames of foreman_cif.y4m.

a) gcc (Ubuntu 7.2.0-8ubuntu3.2) 7.2.0
b) CPU: Intel(R) Core(TM) i7-6900K CPU @ 3.20GHz
c) Config cmd
cmake ../ -DENABLE_CCACHE=1 -DCONFIG_LOWBITDEPTH=1
d) Test cmd:
./aomenc --cpu-used=1 --end-usage=vbr \
--target-bitrate=800 --limit=10

Change-Id: If27543ba53eb1946d0e79d2977a24383377abc71