go/types: re-use type hashing logic in Context.typeHash

The special handling for *Named types is not necessary. The hash of an
instance is simply the hash of its type followed by its type argument
list.

Change-Id: I7aa58e73b81731c3cad3a2fd14124f63cfb685a7
Reviewed-on: https://go-review.googlesource.com/c/go/+/362800


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>