cmd/compile: split Addrconst out of Naddr

cmd/compile: split Addrconst out of Naddr

There are only three Prog types that we were creating with an OLITERAL
Node: ATEXT, ATYPE, and AFUNCDATA. ATEXT's value we later overwrite in
defframe, and ATYPE's we don't even need. AFUNCDATA only needs integer
constants, so get rid of all the non-int constant logic and skip
creating a Node representation for the constant.

While here, there are a few other Naddr code paths that are no longer
needed, so turn those into Fatalfs.

Passes toolstash/buildall.

Change-Id: I4cc9b92c3011890afd4f31ebeba8b1b42b753cab
Reviewed-on: https://go-review.googlesource.com/30074
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>