[dev.regabi] cmd/compile: clean up tests to know less about Node

We want to refactor a bit, and these tests know too much about
the layout of Nodes. Use standard constructors instead.

Change-Id: I91f0325c89ea60086655414468c53419ebeacea4
Reviewed-on: https://go-review.googlesource.com/c/go/+/272626


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>