Stop ICEing on impossible predicates.

Author: cjgillot

compiler/rustc_mir_dataflow/src/value_analysis.rs

@@ -846,9 +846,10 @@ impl Map {
 
         if let ty::Ref(_, ref_ty, _) | ty::RawPtr(ref_ty, _) = ty.kind()
             && let ty::Slice(..) = ref_ty.kind()
+            // The user may have written a predicate like `[T]: Sized` in their where clauses,
+            // which makes slices scalars.
+            && self.places[place].value_index.is_none()
         {
-            assert!(self.places[place].value_index.is_none(), "slices are not scalars");
-
             // Prepend new child to the linked list.
             let len = self.places.push(PlaceInfo::new(Some(TrackElem::DerefLen)));
             self.places[len].next_sibling = self.places[place].first_child;