cmd/compile: treecopy Name

Gc crashes on the following program:

``` go
package a
import "fmt"
const n = fmt
```

```
treecopy [0xc820209d40]
.   PACK l(2) a.fmt
tmp.go:3: internal compiler error: treecopy Name
```

This is a regression from Go1.4, it produced a sensible error message.

go version devel +514014c Thu Jun 18 15:54:35 2015 +0200 linux/amd64
