cmd/compile: change Func.{Enter,Exit} from NodeList to slice

cmd/compile: change Func.{Enter,Exit} from NodeList to slice

Introduces a new types Nodes that can be used to replace NodeList.

Update #14473.

Change-Id: Id77c5dcae0cbeb898ba12dd46bd400aad408871c
Reviewed-on: https://go-review.googlesource.com/19969
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>