If we need to jit the Invoke method of a delegate, we get tramp_info with a
NULL method.

Background: normally when we create a delegate around a virtual method,
handle_delegate_ctor will just create a virtual invoke trampoline with
mono_arch_get_delegate_virtual_invoke_impl which doesn't get here.  But if
we're asked to compile the delegate's Invoke method, then compile_special ()
will create a tramp_info with a null method, and return a delegate trampoline.

That's the case here - we had
  var del = SomeDelegate(obj.VirtualMethod);
  var invoke_method = del.GetType().GetMethod ("Invoke");
  invoke_method.Invoke (del, args);
or
  var del = SomeDelegate(obj.VirtualMethod);
  var another_del = OtherDelegate (del.Invoke);
  another_del (args);

in both cases, we end up in mono_delegate_trampoline with tramp_info->method ==
NULL.

in the second case the IL is like this:
   newobj instance void Derived::'.ctor'
   ldvirtftn instance void class Base::VirtualMethod()
   newobj instance void class SomeDelegate::'.ctor'(object, native int)

So delegate->target is a derived instance but delegate->method is some base
class method.

Addresses mono#17718