Port of dc2ae517 for AVX-2
from Kyle Siefring.

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x86_64:
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cdef_filter_4x4_8bpc_ssse3: 141.7
cdef_filter_4x4_8bpc_sse4: 128.3
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cdef_filter_4x8_8bpc_ssse3: 253.4
cdef_filter_4x8_8bpc_sse4: 228.5
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cdef_filter_8x8_8bpc_ssse3: 429.6
cdef_filter_8x8_8bpc_sse4: 379.9
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x86_32:
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cdef_filter_4x4_8bpc_ssse3: 184.3
cdef_filter_4x4_8bpc_sse4: 168.9
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cdef_filter_4x8_8bpc_ssse3: 335.3
cdef_filter_4x8_8bpc_sse4: 305.1
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cdef_filter_8x8_8bpc_ssse3: 579.1
cdef_filter_8x8_8bpc_sse4: 517.0
------------------------------------------