x86: Add 6-tap variants of high bit-depth mc AVX2 functions

Checkasm numbers on Zen 4:

          8-tap    6-tap
w2_v       20.1     17.4
w2_hv      35.2     32.0

w4_v       21.5     19.1
w4_hv      38.1     34.7

w8_h       33.3     28.4
w8_v       31.3     27.3
w8_hv      91.2     70.1

w16_h      76.1     61.5
w16_v      79.4     69.1
w16_hv    239.7    185.5

w32_h     236.5    197.2
w32_v     237.6    209.1
w32_hv    715.5    559.3

w64_h     820.1    658.0
w64_v     868.3    752.0
w64_hv   2460.1   1952.6

w128_h   2316.8   1860.8
w128_v   2534.2   2201.6
w128_hv  6861.0   5539.8