cmd/compile: internal compiler error: plain block b3 len(Succs)==2, want 1

on dev.ssa branch
go version devel +be2a3e2 Thu Jul 2 08:29:33 2015 +0000 linux/amd64

compile crashes on the following program:

``` go
package a
func f_ssa(){_:_:}
```

```
go.go:2: internal compiler error: plain block b3 len(Succs)==2, want 1
```
