go/types, types2: exclude untyped nil arguments early in type inference

go/types, types2: exclude untyped nil arguments early in type inference

An untyped nil argument cannot be used to infer any type information.
We don't need to include it in the untyped arguments.

Change-Id: Ied44738ff1b135e65a3acfa19223cd3889b7fa7d
Reviewed-on: https://go-review.googlesource.com/c/go/+/492695
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>