gccgo: invalid error message for valid conversion to complex64

gccgo rejects the following program:

``` go
package a
var a = complex64(6)/774516156475830989403545856475830078125
```

```
go.go:2:22: error: complex real part overflow
```

This program is legal and should be compiled without error messages.
See Robert explanation in #11590.

gcc version 6.0.0 20150702 (experimental) (GCC)
