slices: simplify rotate code

slices: simplify rotate code

The rotate-by-reverse code in fact does only 2 writes per entry, so
it is fine and simpler.

Change-Id: I5feea9698b5575f1f0ae9069cc1d074643529262
Reviewed-on: https://go-review.googlesource.com/c/go/+/562321
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>