types.Identical will report that two instantiated types are different
even if they are instantiated with the same type arguments.
Change our comparison so that we treat them as the same.
This lets us drop fully resolving all types, which is good because
that wasn't correct anyhow.
Change-Id: Ibcf2dc08896ecacb11ca302627e5d0c6f0722da7
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go2-dev/+/769080
Reviewed-by: Ian Lance Taylor <iant@google.com>