InvalidOperationException in StackOptimizerPass1.IsIndirectAssignment with `x! = null;`

```c#
class Program
{
    static void Main()
    {
        object x;
        x! = null;
    }
}
```
```
System.InvalidOperationException: Unexpected value 'PassByCopy' 
of type 'Microsoft.CodeAnalysis.CSharp.BoundKind'
   at Microsoft.CodeAnalysis.CSharp.CodeGen.StackOptimizerPass1.IsIndirectAssignment(…)
```