[dev.regabi] cmd/compile: rewrite some generic ir.Nod calls

[dev.regabi] cmd/compile: rewrite some generic ir.Nod calls

An automated rewrite is going to remove the bulk of the calls
to ir.Nod and friends. This CL takes care of the ones that don't
have fixed opcodes and so aren't amenable to automatic rewriting.

Passes buildall w/ toolstash -cmp.

Replay of CL 275886, lost to the bad-merge history rewrite.

Change-Id: I5bf8d1d182f847f4ab44b7e278b752913e30e4c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/277956
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>