add is_sized method on Abi and Layout, and use it

compiler/rustc_const_eval/src/interpret/operand.rs

@@ -280,7 +280,7 @@ impl<'tcx, Prov: Provenance> OpTy<'tcx, Prov> {
         layout: TyAndLayout<'tcx>,
         cx: &impl HasDataLayout,
     ) -> InterpResult<'tcx, Self> {
-        assert!(!layout.is_unsized());
+        assert!(layout.is_sized());
         self.offset_with_meta(offset, MemPlaceMeta::None, layout, cx)
     }
 }