go/types: int overflow in switch expression

gotype successfully compiles the following program:

``` go
package a
func f() {
    switch 9223372036854775808 {
    }
}
```

gc rejects it with:

```
go.go:3: constant 9223372036854775808 overflows int
```

gc seems to be right.

on commit af817890aa7b628234075a6cb01a3a435fb8317d
