[dev.regabi] cmd/compile: simplify Op, Node, Nodes printing

[dev.regabi] cmd/compile: simplify Op, Node, Nodes printing

nconvFmt calls base.Fatalf if mode is anything but FErr,
proving that the only formats that matter for nodes are
plain %v, %S, and %L. And the nodes formatter can only get to %v.
(%S and %v are the same; we'll clean that up separately.)

Node and Nodes can therefore ignore mode, and all the mode
code can be removed from those implementations, removing
quite a few layers of abstraction.

Op similarly only runs in one mode and can be simplified.

Passes buildall w/ toolstash -cmp.

Change-Id: Ibfd845033e9c68181a20fb81c8f3dd428463920a
Reviewed-on: https://go-review.googlesource.com/c/go/+/275775
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>