x/crypto/blake2b: very low performance for AVX and AVX2 code

[aead]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

1.7.*

What operating system and processor architecture are you using (go env)?

amd64/linux on Intel(R) Xeon(R) CPU E5-2620 v3 @ 2.40GHz
Further info

What did you do?

go test -bench=Benchmark for x/crypto/blake2b

What did you expect to see?

BenchmarkWrite128-12              500000              2084 ns/op          61.40 MB/s
BenchmarkWrite1K-12               100000             16227 ns/op          63.10 MB/s
BenchmarkSum128-12               1000000              2107 ns/op          60.74 MB/s
BenchmarkSum1K-12                 100000             16312 ns/op          62.77 MB/s
PASS
ok      golang.org/x/crypto/blake2b     7.647s

What did you see instead?

Performance about 800 MB/s as on a i7-6500U

[aead]

Replacing PINSRQ through VPINSERQ and adding VZEROUPPER doesn't change anything...

[gopherbot]

CL https://golang.org/cl/34993 mentions this issue.