LibJS: Make Optional<Operand> use less space

Shrink these down from 12 bytes to 8 bytes, which helps make many
bytecode instructions smaller.

Author: awesomekling

Libraries/LibJS/Bytecode/Operand.h

@@ -14,6 +14,7 @@ namespace JS::Bytecode {
 class Operand {
 public:
     enum class Type {
+        Invalid,
         Register,
         Local,
         Constant,
@@ -46,3 +47,102 @@ class Operand {
 };
 
 }
+
+namespace AK {
+template<>
+class Optional<JS::Bytecode::Operand> : public OptionalBase<JS::Bytecode::Operand> {
+    template<typename U>
+    friend class Optional;
+
+public:
+    using ValueType = JS::Bytecode::Operand;
+
+    Optional() = default;
+
+    template<SameAs<OptionalNone> V>
+    Optional(V) { }
+
+    Optional(Optional<JS::Bytecode::Operand> const& other)
+    {
+        if (other.has_value())
+            m_value = other.m_value;
+    }
+
+    Optional(Optional&& other)
+        : m_value(other.m_value)
+    {
+    }
+
+    template<typename U = JS::Bytecode::Operand>
+    requires(!IsSame<OptionalNone, RemoveCVReference<U>>)
+    explicit(!IsConvertible<U&&, JS::Bytecode::Operand>) Optional(U&& value)
+    requires(!IsSame<RemoveCVReference<U>, Optional<JS::Bytecode::Operand>> && IsConstructible<JS::Bytecode::Operand, U &&>)
+        : m_value(forward<U>(value))
+    {
+    }
+
+    template<SameAs<OptionalNone> V>
+    Optional& operator=(V)
+    {
+        clear();
+        return *this;
+    }
+
+    Optional& operator=(Optional const& other)
+    {
+        if (this != &other) {
+            clear();
+            m_value = other.m_value;
+        }
+        return *this;
+    }
+
+    Optional& operator=(Optional&& other)
+    {
+        if (this != &other) {
+            clear();
+            m_value = other.m_value;
+        }
+        return *this;
+    }
+
+    void clear()
+    {
+        m_value = JS::Bytecode::Operand { JS::Bytecode::Operand::Type::Invalid, 0 };
+    }
+
+    [[nodiscard]] bool has_value() const
+    {
+        return m_value.type() != JS::Bytecode::Operand::Type::Invalid;
+    }
+
+    [[nodiscard]] JS::Bytecode::Operand& value() &
+    {
+        VERIFY(has_value());
+        return m_value;
+    }
+
+    [[nodiscard]] JS::Bytecode::Operand const& value() const&
+    {
+        VERIFY(has_value());
+        return m_value;
+    }
+
+    [[nodiscard]] JS::Bytecode::Operand value() &&
+    {
+        return release_value();
+    }
+
+    [[nodiscard]] JS::Bytecode::Operand release_value()
+    {
+        VERIFY(has_value());
+        JS::Bytecode::Operand released_value = m_value;
+        clear();
+        return released_value;
+    }
+
+private:
+    JS::Bytecode::Operand m_value { JS::Bytecode::Operand::Type::Invalid, 0 };
+};
+
+}