cmd/compile: make Afunclit the default/only behavior for Naddr

cmd/compile: make Afunclit the default/only behavior for Naddr

Naddr used to translate PFUNC Nodes into references to the function
literal wrapper, and then Afunclit could be used to rewrite it to
reference the function text itself. But now everywhere we use Naddr on
PFUNC Nodes, we immediately call Afunclit anyway. So just merge
Afunclit's behavior into Naddr.

Passes toolstash/buildall.

Change-Id: If2ca6d7f314c1a0711df9b8209aace16ba4b8bc0
Reviewed-on: https://go-review.googlesource.com/30073
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>