[dev.regabi] cmd/compile: rewrite problematic use of Node fields

For the upcoming rewrite to access methods, a few direct accesses
are problematic for the automated tool, most notably direct copies
or use of Node structs as opposed to pointers.

Fix these manually.

Passes toolstash -cmp.

Change-Id: I8bdbb33216737c09e1edda284d5c414422d86284
Reviewed-on: https://go-review.googlesource.com/c/go/+/273006


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