cmd/compile: mis-calculates a constant

For the following program:

``` go
package main
var f = 7 / "0"[0]
```

`go tool compile` says:

```
tmp.go:2: constant 1197 overflows byte
```

Now sure where 1197 came from, but it must be 0 and the program must compile successfully.

go version devel +514014c Thu Jun 18 15:54:35 2015 +0200 linux/amd64
