cmd/compile: generate type equal func during walk

cmd/compile: generate type equal func during walk

So we don't generate ONAME node with nil Func.

Do not pass toolstash-check because the CL changes the order of
compiling functions.

Change-Id: Ib967328f36b8c59a5525445667103c0c80ccdc82
Reviewed-on: https://go-review.googlesource.com/c/go/+/436436
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>