Make sure we generate the right code for zeroing a structure.

Check in after Matthew's CL (34564).

Update #18370

Change-Id: I987087f979d99227a880b34c44d9d4de6c25ba0c
Reviewed-on: https://go-review.googlesource.com/34565Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>