Fix self-assignment of useAVX variable. (vet warning) Change-Id: I4194d7640f17cbf71dae27ce271d6f6d21354318 Reviewed-on: https://go-review.googlesource.com/43371 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org> Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org>