cmd/compile: does not diagnose constant division by zero

gc produces an error for c and d, but not for a and b:

``` go
package a
var a = complex64(0)/1e-1000
var b = complex64(0)/1e-47
var c = float32(0)/1e-1000
var d = float32(0)/1e-47
```

```
go.go:4: division by zero
go.go:5: division by zero
```

gotype produces errors for all 4 variables.

go version devel +9b04852 Sat Jul 11 00:08:50 2015 +0000 linux/amd64
