[dev.regabi] cmd/compile: simplify fmt handling of Nodes

The existing code introduces many types in what appears to be an
attempt to avoid allocation when converting formatting argument lists.
Simplify by accepting that allocation is going to happen, especially
when Node itself turns into an interface.

Change-Id: I3c0d45ca01eace4924deb43c0ea7dc6d65943d08
Reviewed-on: https://go-review.googlesource.com/c/go/+/272929


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