x86: Add prep_8tap_scaled AVX2 asm
x86: Add prep_8tap_scaled AVX2 asm

mct_scaled_8tap_regular_w4_8bpc_c: 872.1
mct_scaled_8tap_regular_w4_8bpc_avx2: 125.6
mct_scaled_8tap_regular_w4_dy1_8bpc_c: 886.3
mct_scaled_8tap_regular_w4_dy1_8bpc_avx2: 84.0
mct_scaled_8tap_regular_w4_dy2_8bpc_c: 1189.1
mct_scaled_8tap_regular_w4_dy2_8bpc_avx2: 84.7

mct_scaled_8tap_regular_w8_8bpc_c: 2261.0
mct_scaled_8tap_regular_w8_8bpc_avx2: 306.2
mct_scaled_8tap_regular_w8_dy1_8bpc_c: 2189.9
mct_scaled_8tap_regular_w8_dy1_8bpc_avx2: 233.8
mct_scaled_8tap_regular_w8_dy2_8bpc_c: 3060.3
mct_scaled_8tap_regular_w8_dy2_8bpc_avx2: 282.8

mct_scaled_8tap_regular_w16_8bpc_c: 4335.3
mct_scaled_8tap_regular_w16_8bpc_avx2: 680.7
mct_scaled_8tap_regular_w16_dy1_8bpc_c: 5137.2
mct_scaled_8tap_regular_w16_dy1_8bpc_avx2: 578.6
mct_scaled_8tap_regular_w16_dy2_8bpc_c: 7878.4
mct_scaled_8tap_regular_w16_dy2_8bpc_avx2: 774.6

mct_scaled_8tap_regular_w32_8bpc_c: 17871.9
mct_scaled_8tap_regular_w32_8bpc_avx2: 2954.8
mct_scaled_8tap_regular_w32_dy1_8bpc_c: 18594.7
mct_scaled_8tap_regular_w32_dy1_8bpc_avx2: 2073.9
mct_scaled_8tap_regular_w32_dy2_8bpc_c: 28696.0
mct_scaled_8tap_regular_w32_dy2_8bpc_avx2: 2852.1

mct_scaled_8tap_regular_w64_8bpc_c: 46967.5
mct_scaled_8tap_regular_w64_8bpc_avx2: 7527.5
mct_scaled_8tap_regular_w64_dy1_8bpc_c: 45564.2
mct_scaled_8tap_regular_w64_dy1_8bpc_avx2: 5262.9
mct_scaled_8tap_regular_w64_dy2_8bpc_c: 72793.3
mct_scaled_8tap_regular_w64_dy2_8bpc_avx2: 7535.9

mct_scaled_8tap_regular_w128_8bpc_c: 111190.8
mct_scaled_8tap_regular_w128_8bpc_avx2: 19386.8
mct_scaled_8tap_regular_w128_dy1_8bpc_c: 122625.0
mct_scaled_8tap_regular_w128_dy1_8bpc_avx2: 15376.1
mct_scaled_8tap_regular_w128_dy2_8bpc_c: 197120.6
mct_scaled_8tap_regular_w128_dy2_8bpc_avx2: 21871.0

xiph/rav1e,xiph/rav1e