intrusive_list: Add size querying

Since we store pointers and have an interface for iterators
set up, the count is just the distance from the beginning
to the end of the list.

Nice thing is that because of this, basic blocks also get
the ability to have a size count without needing to do anything
directly.

src/common/intrusive_list.h

@@ -216,6 +216,14 @@ public:
         return root->next == root.get();
     }
 
+    /**
+     * Gets the total number of elements within this list.
+     * @return the number of elements in this list.
+     */
+    size_type size() const {
+        return static_cast<size_type>(std::distance(begin(), end()));
+    }
+
     /**
      * Retrieves a reference to the node at the front of the list.
      * @note Must not be called on an empty list.