Sigil incorrectly fails when calling a virtual method on a struct

[benjamin-hodgson]

Found this while debugging kevin-montrose/Jil#257. (Jil emits virtual calls to IEnumerable<T>.GetEnumerator.)

public struct MyStruct { }
string Example(MyStruct s) => s.ToString();

Looking at the above method in dotPeek, I see a constrained callvirt on the address of the MyStruct argument:

IL_0000: ldarga.s     s
IL_0002: constrained. MyStruct
IL_0008: callvirt     instance string [mscorlib]System.Object::ToString()
IL_000d: ret

Here's (what I think is) the equivalent Sigil:

var obj = typeof(object);
var myStruct = typeof(MyStruct);
var myDelegate = Emit<Func<MyStruct, string>>.NewDynamicMethod()
    .LoadArgumentAddress(0)
    .CallVirtual(obj.GetMethod("ToString"), myStruct)
    .Return()
    .CreateDelegate();

This fails with a SigilVerificationException.

Sigil.SigilVerificationException: CallVirtual expected a System.Object; found MyStruct*
   at Sigil.Emit`1.UpdateStackAndInstrStream(Nullable`1 instr, TransitionWrapper transitions, Boolean firstParamIsThis)
   at Sigil.Emit`1.UpdateState(OpCode instr, MethodInfo method, IEnumerable`1 parameterTypes, TransitionWrapper transitions, Boolean firstParamIsThis, Type[] arglist)
   at Sigil.Emit`1.CallVirtual(MethodInfo method, Type constrained, Type[] arglist)
   at Program.Main(String[] args) in Program.cs:line 18