cmd/compile: factor out code to generate equal func

cmd/compile: factor out code to generate equal func

So next CL can use it for generating equal func during walk compare.

Passes toolstash-check.

Change-Id: I76545c1d471eb496be352908db1b05feae83fc33
Reviewed-on: https://go-review.googlesource.com/c/go/+/436435
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>