LibJS: Add basic support for while loops in the bytecode engine

This introduces two new instructions: Jump and JumpIfFalse.
Jumps are made to a Bytecode::Label, which is a simple object that
represents a location in the bytecode stream.

Note that you may not always know the target of a jump when adding the
jump instruction itself, but we can just update the instruction later
on during codegen once we know where the jump target is.

The Bytecode::Interpreter now implements jumping via a jump slot that
gets checked after each instruction to see if a jump is pending.
If not, we just increment the PC as usual.

Userland/Libraries/LibJS/Bytecode/Op.h

@@ -8,6 +8,7 @@
 
 #include <AK/FlyString.h>
 #include <LibJS/Bytecode/Instruction.h>
+#include <LibJS/Bytecode/Label.h>
 #include <LibJS/Bytecode/Register.h>
 #include <LibJS/Heap/Cell.h>
 #include <LibJS/Runtime/Value.h>
@@ -139,4 +140,38 @@ private:
     FlyString m_identifier;
 };
 
+class Jump final : public Instruction {
+public:
+    explicit Jump(Label target)
+        : m_target(target)
+    {
+    }
+
+    virtual ~Jump() override { }
+    virtual void execute(Bytecode::Interpreter&) const override;
+    virtual String to_string() const override;
+
+private:
+    Label m_target;
+};
+
+class JumpIfFalse final : public Instruction {
+public:
+    explicit JumpIfFalse(Register result, Optional<Label> target = {})
+        : m_result(result)
+        , m_target(move(target))
+    {
+    }
+
+    void set_target(Optional<Label> target) { m_target = move(target); }
+
+    virtual ~JumpIfFalse() override { }
+    virtual void execute(Bytecode::Interpreter&) const override;
+    virtual String to_string() const override;
+
+private:
+    Register m_result;
+    Optional<Label> m_target;
+};
+
 }