[dev.regabi] cmd/compile: use ir.Find for "search" traversals

[dev.regabi] cmd/compile: use ir.Find for "search" traversals

This CL converts all the generic searching traversal to use ir.Find
instead of relying on direct access to Left, Right, and so on.

Passes buildall w/ toolstash -cmp.

Change-Id: I4d951aef630c00bf333f24be79565cc564694d04
Reviewed-on: https://go-review.googlesource.com/c/go/+/275372
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>