Includes unit tests for conformance and speed.

SSSE3/CFLPredictHBDTest:
4x4: C time = 1436 us, SIMD time = 358 us (~4x)
8x8: C time = 4821 us, SIMD time = 598 us (~8.1x)
16x16: C time = 18528 us, SIMD time = 1793 us (~10x)
32x32: C time = 72998 us, SIMD time = 6400 us (~11x)

AVX2/CFLPredictHBDTest:
4x4: C time = 1436 us, SIMD time = 398 us (~3.6x)
8x8: C time = 4924 us, SIMD time = 644 us (~7.6x)
16x16: C time = 18624 us, SIMD time = 1617 us (~12x)
32x32: C time = 73509 us, SIMD time = 3635 us (~20x)

Change-Id: Icbcfefbf165facdbd77c9b3861af2bbf464254a0