cmd/compile: missing "out of bounds" error slicing [0]T

```go
package p

func f() {
	var b [0]struct{}
	_ = b[2:]
}
```

gotype says:

```
x.go:5:8: index 2 (constant of type int) is out of bounds
```

cmd/compile gives no error. If you change b's type to `[1]struct{}`, the error from cmd/compile is:

```
x.go:5: invalid slice index 2 (out of bounds for 1-element array)
```

Reproduces with (at least) 1.7 and 1.8 and 1.9beta1.
