followup to [25d3713b07a100d8fdb349317db97fd9d0c1e366]
Resolves #12215

Previous code didn't account for the extra unreach() that now exists in the air:

```
 %29!= block(void, {
    %30!= cond_br(%22!, {
      %31!= br(%29, @Air.Inst.Ref.void_value)
    }, {
      %2! %15!
      %27!= call(%26, [%19!, %21])
      %28!= unreach()
    })
  } %22!)
```