[dev.typeparams] cmd/compile: allow types with the same underlying type to have the same shape

First baby step to sharing the underlying implementation among several types.

Change-Id: I6a156176d2b7f0131a87285a03b881ce380c26ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/338610


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