cmd/compile: implement range over integer

cmd/compile: implement range over integer

Add compiler implementation of range over integers.
This is only reachable if GOEXPERIMENT=range is set,
because otherwise type checking will fail.

For proposal #61405 (but behind a GOEXPERIMENT).
For #61717.

Change-Id: I4e35a73c5df1ac57f61ffb54033a433967e5be51
Reviewed-on: https://go-review.googlesource.com/c/go/+/510538
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>