cmd/compile/internal/types2: implement deduplication of instances using the Environment

This is a port of CL 344390 with adjustments to names to make it
work for types2.

Change-Id: I05c33d9858f973adfbf48d8a1faaf377280f6985
Reviewed-on: https://go-review.googlesource.com/c/go/+/348572


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