[dev.regabi] cmd/compile: use ir.EditChildren for inline rewriting

[dev.regabi] cmd/compile: use ir.EditChildren for inline rewriting

This CL rephrases the general inlining rewriter in terms of ir.EditChildren.
It is the final part of the code that was processing arbitrary nodes using
Left, SetLeft, and so on. After this CL, there should be none left except
for the implementations of DoChildren and EditChildren, which fall next.

Passes buildall w/ toolstash -cmp.

Change-Id: I9c36053360cd040710716f0b39397a80114be713
Reviewed-on: https://go-review.googlesource.com/c/go/+/275373
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>