Improve vp9_iht4x4_16_add_sse2 (x1.341)

This rebase is a better implementation of the previous ones.

Modifications are done to reduce the total clock cycle.
Speedup: 1.341
Compiled with -O3
Tested with: park_joy_420_720p50.y4m

Change-Id: I940eaf283f60597ca0d9d2e13d518878d55ff02d