[dev.regabi] cmd/compile: cleanup noder

Similar to previous CL: take advantage of better constructor APIs for
translating ASTs from syntax to ir.

Passes toolstash -cmp.

Change-Id: I40970775e7dd5afe2a0b7593ce3bd73237562457
Reviewed-on: https://go-review.googlesource.com/c/go/+/279972


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>