[dev.typeparams] cmd/compile/internal/types: review of return.go

The changes between (equivalent, and reviewed) go/types/return.go
and return.go can be seen by comparing patchset 1 and 2. The actual
change is just removing the "// UNREVIEWED" marker.

Change-Id: I7bb3201abec75043804296d6c37307fd243d58f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/291175


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>