x/tools/go/types: compiles invalid program with overflow

On the following program:

``` go
package main
func main() {
    var a = -("0"[0])
    println(a)
}
```

`ssadump -run` outputs:

```
208
```

The program is invalid and must not be compiled (type of "0"[0] is byte, -byte is an overflow).

on commit e9a746de6e61b9b823a981843c0d98bc209eed2b
