Node.Addrtaken is treated as a bool, so make it a bool.

I'll start to batch these changes if they are simple.

Change-Id: I02a3d1131efc4e12b78b83372c1b50f8b160c194
Reviewed-on: https://go-review.googlesource.com/6911Reviewed-by: Ian Lance Taylor <iant@golang.org>