small tweak for read_from_term performance (#2668)

src/machine/loader.rs

@@ -1384,10 +1384,16 @@ impl MachineState {
             stackful_post_order_iter::<NonListElider>(&mut self.heap, &mut self.stack, 0);
 
         while let Some(addr) = iter.next() {
-            let addr = unmark_cell_bits!(heap_bound_store(
+            let addr = unmark_cell_bits!(addr);
+
+            let addr = if addr.is_var() {
+                unmark_cell_bits!(heap_bound_store(
                     iter.base_iter.heap,
-                heap_bound_deref(iter.base_iter.heap, addr),
-            ));
+                    heap_bound_deref(iter.base_iter.heap, addr)
+                ))
+            } else {
+                addr
+            };
 
             read_heap_cell!(addr,
                 (HeapCellValueTag::Lis) => {