Rollup merge of rust-lang#146564 - cjgillot:mir-nolen, r=scottmcm

Remove Rvalue::Len again.

Now that we have `RawPtrKind::FakeForPtrMetadata`, we can reimplement `Rvalue::Len` using `PtrMetadata(&raw const (fake) place)`.

r? ``@scottmcm``

Author: Zalathar

compiler/rustc_public/src/unstable/convert/stable/mir.rs

@@ -215,7 +215,6 @@ impl<'tcx> Stable<'tcx> for mir::Rvalue<'tcx> {
                 mutability.stable(tables, cx),
                 place.stable(tables, cx),
             ),
-            Len(place) => crate::mir::Rvalue::Len(place.stable(tables, cx)),
             Cast(cast_kind, op, ty) => crate::mir::Rvalue::Cast(
                 cast_kind.stable(tables, cx),
                 op.stable(tables, cx),