Improve AVX2 distance computations (#2252)
Improve AVX2 distance computations (#2252)

Summary:
Pull Request resolved: https://github.com/facebookresearch/faiss/pull/2252

Improve fvec_op_ny_D4 for AVX2.

Reviewed By: mdouze

Differential Revision: D34729914

fbshipit-source-id: ac51eb3804a9df9c0ec56963f477ddc70e44d8fa

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