The decomposer of builtin types is confused by having structs
still around from the user-type decomposer.  They're all dead though,
so just enabling a deadcode pass fixes things.

Change-Id: I2df6bc7e829be03eabfd24c8dda1bff96f3d7091
Reviewed-on: https://go-review.googlesource.com/21839
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>