Node.Needzero only has two values and acts as a bool, so make it a bool.

Change-Id: Ica46e5ebafbe478017ea52ce6bb335f404059677
Reviewed-on: https://go-review.googlesource.com/6800Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>