go/types: don't write during sanitizeInfo if nothing has changed

In its final phase, the typechecker walks the types it produces to
ensure that no unexpanded type instances leak through the API. However,
this also walks shared types (such as those in the universe scope),
resulting in a potential data race during concurrent typechecking
passes.

Fix this by being careful not to write if nothing needs to be changed.
Since any shared types should already be sanitized, this should
eliminate data races.

For #44434

Change-Id: Iadb2e78863efe0e974e69a00e255f26cfaf9386a
Reviewed-on: https://go-review.googlesource.com/c/go/+/294411


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