compiler: explicitly convert between type aliases

Otherwise we can get a crash in the backend.

Test case is https://golang.org/cl/73790.

Change-Id: I78e4c962d63308c971ad35b9f92b8de30f4d413e
Reviewed-on: https://go-review.googlesource.com/73810
Reviewed-by: Than McIntosh <thanm@google.com>
1 file changed