[dev.regabi] cmd/compile: move func code from node.go to func.go

[dev.regabi] cmd/compile: move func code from node.go to func.go

No code changes here, only copying of text.
This will make the diffs in a future CL readable.

Passes buildall w/ toolstash -cmp.

Change-Id: I325a62e79edd82f1437769891ea63a32f51c0170
Reviewed-on: https://go-review.googlesource.com/c/go/+/274095
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>