cmd/compile: failure to diagnose invalid conversion to complex64

gc successfully compiles the following program:

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

while both gccgo and go/types reject it with:

```
error: complex real part overflow
774516156475830989403545856475830078125 (untyped int constant) overflows complex64
```

go version devel +40a1516 Fri Jul 3 03:08:40 2015 +0000 linux/amd64
