On amd64:

benchmark           old ns/op    new ns/op    delta
BenchmarkEncrypt         6170         3593  -41.77%
BenchmarkDecrypt         6209         3564  -42.60%

benchmark            old MB/s     new MB/s  speedup
BenchmarkEncrypt         1.30         2.23    1.72x
BenchmarkDecrypt         1.29         2.24    1.74x

Update #4299.

R=golang-dev, agl, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/11874043