Speed Up SADNxNx4D

[luctrudeau]

More than 15% of the encoding time of libVPX on POWER is spent in the SADNxNx4D functions.

%	Function
10.63%	vpx_sad16x16x4d_vsx
3.60%	vpx_sad32x32x4d_vsx
3.22%	vpx_sad64x64x4d_vsx
1.12%	vpx_sad8x8x4d_c

Current VSX SAD implementations can be further optimized for considerable performance improvements. Doubling the speed of the SADNxNx4D functions would reduce encoding time by 5 to 8%.

This includes the following functions:

• [ ] vpx_sad16x16x4d_vsx
• [ ] vpx_sad32x32x4d_vsx
• [ ] vpx_sad64x64x4d_vsx
• [ ] vpx_sad8x8x4d_vsx
• [ ] PROCESS16_4D
• [ ] SAD8_4D
• [ ] SAD16_4D
• [ ] SAD32_4D
• [ ] SAD64_4D

Testing:

• [ ] Must pass the SADx4Test suite
• [ ] Refactor SADx4Test to use the AbstractBench
• [ ] Report performance in commit msg (compared to C version)
• [ ] Show significant speedup over C version