compiler: handle aliases to pointer types with interfaces

Test case is https://golang.org/cl/241997.

Change-Id: Ie30d682e016f45cc29edb4c992eaab40d3339314
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/241998
Reviewed-by: Cherry Zhang <cherryyz@google.com>