cmd/compile: remove unused cases in Naddr

cmd/compile: remove unused cases in Naddr

Gins, and in turn Naddr, is only used with ONAME and OLITERAL Nodes,
so we can drastically simplify Naddr.

Passes toolstash/buildall.

Change-Id: I2deb7eb771fd55e7c7f00040a9aee54588fcac11
Reviewed-on: https://go-review.googlesource.com/29247
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>