On amd64
benchmark                        old ns/op    new ns/op    delta
BenchmarkHashStringSpeed                91           74  -18.49%
BenchmarkHashInt32Speed                 54           45  -17.88%
BenchmarkHashInt64Speed                 76           58  -23.53%
BenchmarkHashStringArraySpeed          231          188  -18.61%

Fixes #5367.

R=golang-dev, iant, dave, daniel.morsing, ality, rsc
CC=golang-dev
https://golang.org/cl/9040043