x86: AVX memzero_aligned
x86: AVX2 nal_escape

Also rewrite the entire function to be faster and drop the AVX version which is no longer useful.
x86: AVX2 high_bit_depth pixel_avg2, get_ref, mc_copy_w16, mc_luma

Also reduce the number of xmm registers used by mc_copy_* to avoid
saving and restoring xmm6 and xmm7 on 64-bit Windows.
x86: AVX2 high bit-depth pixel_sad

Also use loops instead of duplicating code; reduces code size by ~10kB with
negligible effect on performance.
x86: AVX2 high bit-depth vsad
x86: AVX2 high bit-depth pixel_sad_x3/pixel_sad_x4

Also reduce the number of xmm registers used by sse2/ssse3 pixel_sad_x3.
x86: AVX2 high bit-depth pixel_ssd
x86: AVX2 pixel_ssd_nv12_core
x86: SSSE3 ads_mvs

~55% faster ads in benchasm, ~15-30% in real encoding.
~4% faster "placebo" preset overall.
x86: BMI2 cabac_residual functions
x86: SSSE3 LUT-based faster coeff_level_run

~2x faster coeff_level_run.
Faster CAVLC encoding: {1%,2%,7%} overall with {superfast,medium,slower}.
Uses the same pshufb LUT abuse trick as in the previous ads_mvs patch.