GR-43663: implement the computeSize truffle api

- for the truffle node splitting heuristic

Author: cosminbasca

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/nodes/bytecode/PBytecodeRootNode.java

@@ -688,6 +688,11 @@ private PBytecodeRootNode(PythonLanguage language, FrameDescriptor fd, Signature
         this.selfIndex = selfIndexValue;
     }
 
+    @Override
+    protected int computeSize() {
+        return bytecode.length / 2;
+    }
+
     @Override
     public String getName() {
         return name.toJavaStringUncached();