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;

Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp

@@ -105,4 +105,14 @@ Optional<Bytecode::Register> WhileStatement::generate_bytecode(Bytecode::Generat
     return body_result_reg;
 }
 
+Optional<Bytecode::Register> DoWhileStatement::generate_bytecode(Bytecode::Generator& generator) const
+{
+    auto head_label = generator.make_label();
+    auto body_result_reg = m_body->generate_bytecode(generator);
+    auto test_result_reg = m_test->generate_bytecode(generator);
+    VERIFY(test_result_reg.has_value());
+    generator.emit<Bytecode::Op::JumpIfTrue>(*test_result_reg, head_label);
+    return body_result_reg;
+}
+
 }

Userland/Libraries/LibJS/Bytecode/Op.cpp

@@ -64,6 +64,14 @@ void JumpIfFalse::execute(Bytecode::Interpreter& interpreter) const
         interpreter.jump(m_target.value());
 }
 
+void JumpIfTrue::execute(Bytecode::Interpreter& interpreter) const
+{
+    VERIFY(m_target.has_value());
+    auto result = interpreter.reg(m_result);
+    if (result.as_bool())
+        interpreter.jump(m_target.value());
+}
+
 String Load::to_string() const
 {
     return String::formatted("Load dst:{}, value:{}", m_dst, m_value.to_string_without_side_effects());
@@ -116,4 +124,11 @@ String JumpIfFalse::to_string() const
     return String::formatted("JumpIfFalse result:{}, target:<empty>", m_result);
 }
 
+String JumpIfTrue::to_string() const
+{
+    if (m_target.has_value())
+        return String::formatted("JumpIfTrue result:{}, target:{}", m_result, m_target.value());
+    return String::formatted("JumpIfTrue result:{}, target:<empty>", m_result);
+}
+
 }

Userland/Libraries/LibJS/Bytecode/Op.h

@@ -193,4 +193,23 @@ private:
     Optional<Label> m_target;
 };
 
+class JumpIfTrue final : public Instruction {
+public:
+    explicit JumpIfTrue(Register result, Optional<Label> target = {})
+        : m_result(result)
+        , m_target(move(target))
+    {
+    }
+
+    void set_target(Optional<Label> target) { m_target = move(target); }
+
+    virtual ~JumpIfTrue() override { }
+    virtual void execute(Bytecode::Interpreter&) const override;
+    virtual String to_string() const override;
+
+private:
+    Register m_result;
+    Optional<Label> m_target;
+};
+
 }