cmd/compile: refactor generic AST walking code

cmd/compile: refactor generic AST walking code

racewalk's "foreach" function applies a function to all of a Node's
immediate children, but with a non-idiomatic signature.

This CL reworks it to recursively iterate over the entire subtree
rooted at Node and provides a way to short-circuit iteration.

Passes toolstash -cmp for std cmd with -race.

Change-Id: I738b73953d608709802c97945b7e0f4e4940d3f4
Reviewed-on: https://go-review.googlesource.com/71911
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>