[dev.typeparams] cmd/compile/internal/types2: implement ch <- x where ch is of type parameter type

For #47115.

Change-Id: Ib9c8652c0346029369735ccf7ee9098ab1ae7fd3
Reviewed-on: https://go-review.googlesource.com/c/go/+/333712


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>