[dev.regabi] cmd/compile: late expansion of return values

By-hand rebase of earlier CL, because that was easier than
letting git try to figure things out.

This will naively insert self-moves; in the case that these
involve memory, the expander detects these and removes them
and their vardefs.

Change-Id: Icf72575eb7ae4a186b0de462bc8cf0bedc84d3e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/279519


Trust: David Chase <drchase@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>