go/types: minor cleanup of instantiation

This CL addresses a couple TODOs related to instantiation:
 - factor out resolving the best environment
 - don't eagerly resolve substituted instances

Change-Id: I4a5de7ea7939b6f272991071f591d622dec04b53
Reviewed-on: https://go-review.googlesource.com/c/go/+/349429


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