cmd/compile: use of untyped nil in switch

gc fails on the following program:

``` go
package a
func f() {
    switch nil {
    }
}
```

```
go.go:4: use of untyped nil
```

The program is correct and must be compiled without errors.

go version devel +9b04852 Sat Jul 11 00:08:50 2015 +0000 linux/amd64
