Add AVX2 SIMD implementation for cdef_dir
Add AVX2 SIMD implementation for cdef_dir

cdef_dir_8bpc_c: 629.3
cdef_dir_8bpc_avx2: 82.4

First 1000 frames of Chimera 1080p:
before: 0m23.084s
after: 0m21.860s

xiph/rav1e,xiph/rav1e