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