LibJS: Generate bytecode for do...while statements :^)

This was quite straightforward using the same label/jump machinery that
we added for while statements.

The main addition here is a new JumpIfTrue bytecode instruction.

Userland/Libraries/LibJS/AST.h

@@ -391,6 +391,7 @@ public:
 
     virtual Value execute(Interpreter&, GlobalObject&) const override;
     virtual void dump(int indent) const override;
+    virtual Optional<Bytecode::Register> generate_bytecode(Bytecode::Generator&) const override;
 
 private:
     NonnullRefPtr<Expression> m_test;