Avoids allocating an ONAME for OLABEL, OGOTO, and named OBREAK and
OCONTINUE nodes.

Passes toolstash-check.

Change-Id: I359142cd48e8987b5bf29ac100752f8c497261c1
Reviewed-on: https://go-review.googlesource.com/c/145200
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>