Provides over 15x speedup for width > 8.

Due to smaller loads and shifting for width == 8 it gets about 8x
speedup.

For width == 4 it's only about 4x speedup because there is a lot of
shuffling and shifting to get the data properly situated.

BUG=webm:1390

Change-Id: Ice0b3dbbf007be3d9509786a61e7f35e94bdffa8