cmd/compile: convert typecheckdefstack to []*Node

cmd/compile: convert typecheckdefstack to []*Node

This one 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: Ie02d2cf35f1e8438c6e9dc1d5fba51e8adde1bc0
Reviewed-on: https://go-review.googlesource.com/14480
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>