Avoids generating useless equality and hash functions.

Shrinks cmd/go executable size on linux/amd64 by ~17kB.

Change-Id: Ifde5315cc5cbceb3a7260195c8803cace952359f
Reviewed-on: https://go-review.googlesource.com/66650
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>