cmd/compile: add nodeSeqIterator interface

cmd/compile: add nodeSeqIterator interface

I tried to write a program to convert *NodeList to Node, but ran into
too many problem cases.  I'm backing off and trying a more iterative
approach using interfaces.

This CL adds an interface for iteration over either a *NodeList or a
Nodes.  I changed typechecklist to use it, to show how it works.  After
NodeList is eliminated, we can change the typechecklist parameter type
to Nodes.

Passes toolstash -cmp.

Change-Id: I5c7593714b020d20868b99151b1e7cadbbdbc397
Reviewed-on: https://go-review.googlesource.com/20190
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>