cmd/compile: use []*Node instead of NodeList in sinit

cmd/compile: use []*Node instead of NodeList in sinit

This is a first of a set of changes to make the transition away from NodeList
easier by removing cases in which NodeList doesn't act semi-trivially like a
[]*Node.

This CL was originally prepared by Josh Bleecher Snyder <josharian@gmail.com>.

This change passes go build -toolexec 'toolstash -cmp' -a std.

Change-Id: Iad10b75e42b5b24e1694407841282fa3bab2dc9f
Reviewed-on: https://go-review.googlesource.com/14232
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>