[dev.regabi] cmd/compile: implement doChildren for nodes
Put each node in charge of its DoChildren implementation.
This removes a generic use of Left, Right, and so on
in func DoChildren, heading toward removing those even from
being used in package ir.
Passes buildall w/ toolstash -cmp.
Change-Id: Ibdf56f36801217cf24549e063da0078c1820a56b
Reviewed-on: https://go-review.googlesource.com/c/go/+/275375
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Matthew Dempsky <mdempsky@google.com>