cmd/compile: add Node.IsMethod helper

cmd/compile: add Node.IsMethod helper

Changes generated with eg:

func before(n *gc.Node) bool { return n.Type.Recv() != nil }
func after(n *gc.Node) bool  { return n.IsMethod() }

func before(n *gc.Node) bool { return n.Type.Recv() == nil }
func after(n *gc.Node) bool  { return !n.IsMethod() }

Change-Id: I28e544490d17bbdc06ab11ed32464af5802ab206
Reviewed-on: https://go-review.googlesource.com/28968
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>