InvalidOperationException in StackOptimizerPass1.IsIndirectAssignment with x! = null;

[cston]

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(…)

[jcouv]

This was fixed (I think by #32850)
I'll add a test to verify