go/types: string(1 << s) should be an error

```Go
package main

func main() {
	var s uint
	_ = string(1 << s)
}
```
(https://play.golang.org/p/owHsmdZd32v) is an error: The `1` in `1 << s` assumes the type it would have without the shift, which is `string`. Both cm/compile and gccgo correct report an error.

go/types appears to accept it.