gccgo: can shift complex numbers

gccgo successfully compiles the following program:

``` go
package a
var a = complex64(1) << 2
```

gc rejects it saying:

```
invalid operation: complex64(1) << 2 (shift of type complex64)
```

Spec says that lhs of shift must be an integer.
Similar issue for go/types #11594

gcc version 6.0.0 20150702 (experimental) (GCC)
