go/types, types2: use Identical rather than unification in missingMethod

Now that we instantiate methods on instantiated types, there is no need
to use unification to match signatures inside of missingMethod.

Generally, we should never encounter uninstantiated signatures within
statements. If we do encounter signatures that contain type parameters,
it is because the signatures are themselves defined or instantiated
using type parameters declared in the function scope (see example
below). The current unification logic would not handle this.

	type S[T any] struct{}
	func (S[T]) m(T)

	func _[P any]() bool {
		var v interface{m(int)}
		_, ok = v.(S[P])
		return ok
	}

Change-Id: I754fb5535bba2fc7a209dc7419fd4015c413c9a6
Reviewed-on: https://go-review.googlesource.com/c/go/+/379540


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