addressing ref objects and comparing the pointer address in vm give FieldDefect

[ringabout]

type
  SinglyLinkedList[T] = ref object

proc addMoved[T](a, b: var SinglyLinkedList[T]) =
  if a.addr != b.addr: discard

proc main =
  var a: SinglyLinkedList[int]; new a
  var b: SinglyLinkedList[int]; new b
  a.addMoved b

static: main()

gives

field 'node' is not accessible for type 'TFullReg' using 'kind = rkRegisterAddr' [FieldDefect]

Nim version: 1.6.6/devel

I found the issue while working on #19902