message: add is_sized method on Abi and Layout, and use it
diff: @@ -1053,7 +1053,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
  // pointers we need to retag, so we can stop recursion early.
  // This optimization is crucial for ZSTs, because they can contain way more fields
  // than we can ever visit.
  - if !place.layout.is_unsized() && place.layout.size < self.ecx.pointer_size() {
  + if place.layout.is_sized() && place.layout.size < self.ecx.pointer_size() {
  return Ok(());
  }
  