LibJS: Add basic "if" statement support to the bytecode VM :^)

This also required making Bytecode::Op::Jump support lazy linking
to a target label.

I left a FIXME here about having the "if" statement return the result
value from the taken branch statement. That's what the AST interpreter
does but I'm not sure if it's actually required.

Userland/Libraries/LibJS/AST.h

@@ -351,6 +351,7 @@ 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_predicate;

Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp

@@ -187,4 +187,26 @@ Optional<Bytecode::Register> ReturnStatement::generate_bytecode(Bytecode::Genera
     return argument_reg;
 }
 
+Optional<Bytecode::Register> IfStatement::generate_bytecode(Bytecode::Generator& generator) const
+{
+    auto predicate_reg = m_predicate->generate_bytecode(generator);
+    auto& if_jump = generator.emit<Bytecode::Op::JumpIfTrue>(*predicate_reg);
+    auto& else_jump = generator.emit<Bytecode::Op::JumpIfFalse>(*predicate_reg);
+
+    if_jump.set_target(generator.make_label());
+    (void)m_consequent->generate_bytecode(generator);
+    auto& end_jump = generator.emit<Bytecode::Op::Jump>();
+
+    else_jump.set_target(generator.make_label());
+    if (m_alternate) {
+        (void)m_alternate->generate_bytecode(generator);
+    }
+
+    end_jump.set_target(generator.make_label());
+
+    // FIXME: Do we need IfStatement to return the consequent/alternate result value?
+    //        (That's what the AST interpreter currently does)
+    return {};
+}
+
 }

Userland/Libraries/LibJS/Bytecode/Op.cpp

@@ -72,7 +72,7 @@ void PutById::execute(Bytecode::Interpreter& interpreter) const
 
 void Jump::execute(Bytecode::Interpreter& interpreter) const
 {
-    interpreter.jump(m_target);
+    interpreter.jump(*m_target);
 }
 
 void JumpIfFalse::execute(Bytecode::Interpreter& interpreter) const
@@ -196,7 +196,7 @@ String GetById::to_string() const
 
 String Jump::to_string() const
 {
-    return String::formatted("Jump {}", m_target);
+    return String::formatted("Jump {}", *m_target);
 }
 
 String JumpIfFalse::to_string() const

Userland/Libraries/LibJS/Bytecode/Op.h

@@ -214,17 +214,19 @@ private:
 
 class Jump final : public Instruction {
 public:
-    explicit Jump(Label target)
+    explicit Jump(Optional<Label> target = {})
-        : m_target(target)
+        : m_target(move(target))
     {
     }
 
+    void set_target(Optional<Label> target) { m_target = move(target); }
+
     virtual ~Jump() override { }
     virtual void execute(Bytecode::Interpreter&) const override;
     virtual String to_string() const override;
 
 private:
-    Label m_target;
+    Optional<Label> m_target;
 };
 
 class JumpIfFalse final : public Instruction {