runtime: try harder to get different iteration orders.
runtime: try harder to get different iteration orders.

Fixes issue 8736.

LGTM=iant, josharian
R=golang-codereviews, iant, josharian
CC=golang-codereviews
https://codereview.appspot.com/144910044

firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang