Add AVX2 version of generate_grain_uv (4:2:0).
gen_grain_uv_ar0_8bpc_420_c: 46407.4
gen_grain_uv_ar0_8bpc_420_avx2: 6718.4
gen_grain_uv_ar1_8bpc_420_c: 61715.0
gen_grain_uv_ar1_8bpc_420_avx2: 17591.5
gen_grain_uv_ar2_8bpc_420_c: 90880.3
gen_grain_uv_ar2_8bpc_420_avx2: 26969.6
gen_grain_uv_ar3_8bpc_420_c: 127393.7
gen_grain_uv_ar3_8bpc_420_avx2: 30414.7
Edited by Ronald S. Bultje