cmd/compile: unnecessary error after "cannot refer to unexported type"

https://play.golang.org/p/ZwFSxT9JCu

Output on both 1.9.2 and tip:

```
tmp/sandbox627872070/main.go:6:8: cannot refer to unexported name bytes.buffer
tmp/sandbox627872070/main.go:6:8: undefined: bytes.buffer
```

Is the second error really necessary? It seems redundant to me, unless there is a good reason for it to be there.

/cc @mdempsky @griesemer 