go/types, types2: call recordInstance in instantiateSignature

go/types, types2: call recordInstance in instantiateSignature

This matches the pattern we use for type instantiations and
factors out the recordInstance and associated assert calls.

Change-Id: Ib7731c0e619aca42f418cb2d9a153785aaf014cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/494457
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>