gccgo: can shift by complex number

gccgo successfully compiles the following program:

``` go
package a
var a = 0 << complex64(1)
```

go/types say:

```
go.go:2: invalid operation: 0 << complex64(1) (shift count type complex64, must be unsigned integer)
```

gccgo must unlearn how to shift by complex numbers.

https://github.com/golang/go/issues/11567
