x86: more AVX2 framework, AVX2 functions, plus some existing asm tweaks

AVX2 functions:
mc_chroma
intra_sad_x3_16x16
last64
ads
hpel
dct4
idct4
sub16x16_dct8
quant_4x4x4
quant_4x4
quant_4x4_dc
quant_8x8
SAD_X3/X4
SATD
var
SSD
zigzag interleave
weightp
weightb
intra_sad_8x8_x9
decimate
integral
hadamard_ac
sa8d_satd
sa8d
lowres_init
denoise
x86util: Support ymm registers in HADD macros
x86: AVX2 high bit-depth predict_8x8c_h/predict_8x16c_h
x86: AVX2 high bit-depth predict_16x16_h
x86: AVX2 high bit-depth predict_4x4_h
x86: AVX high bit-depth predict_16x16_v

Also restructure some code to reduce code size of various functions,
especially in high bit-depth.
x86: AVX2 predict_16x16_p

Also fix the AVX implementation to correctly use the SSSE3 inline asm
instead of SSE2.
x86: AVX2 predict_8x8c_p/predict_8x16c_p
x86: AVX2 predict_16x16_dc
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.