[dev.typeparams] cmd/compile: add CONVIFACE nodes for return values during noder2

Even if we can otherwise transform a return statement because of type
params, add CONVIFACE nodes where appropriate.

Change-Id: Ia2216d5f6805926075ba6802a4385eee1d63e37e
Reviewed-on: https://go-review.googlesource.com/c/go/+/337049


Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>