JSSpecCompiler: Prepare for building SSA

This commit introduces NamedVariableDeclaration and
SSAVariableDeclaration and allows storing both of them in Variable node.
Also, it adds additional structures in FunctionDefinition and
BasicBlock, which will be used to store SSA form related information.

Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Compiler/EnableGraphPointers.h

@@ -0,0 +1,69 @@
+/*
+ * Copyright (c) 2023, Dan Klishch <danilklishch@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NumericLimits.h>
+#include <AK/Vector.h>
+
+namespace JSSpecCompiler {
+
+struct VoidRef { };
+
+template<typename T, typename NativeNodeRef = VoidRef>
+class EnableGraphPointers {
+public:
+    class VertexBase {
+    public:
+        VertexBase() = default;
+        VertexBase(size_t index)
+            : m_index(index)
+        {
+        }
+
+        bool is_invalid() const { return m_index == invalid_node; }
+        operator size_t() const { return m_index; }
+
+        explicit VertexBase(NativeNodeRef const& node)
+        requires(!IsSame<NativeNodeRef, VoidRef>)
+            : VertexBase(node->m_index)
+        {
+        }
+
+        auto& operator*() const { return m_instance->m_nodes[m_index]; }
+        auto* operator->() const { return &m_instance->m_nodes[m_index]; }
+
+    protected:
+        size_t m_index = invalid_node;
+    };
+
+    using Vertex = VertexBase;
+
+    inline static constexpr size_t invalid_node = NumericLimits<size_t>::max();
+
+    template<typename Func>
+    void with_graph(Func func)
+    {
+        m_instance = static_cast<T*>(this);
+        func();
+        m_instance = nullptr;
+    }
+
+    template<typename Func>
+    void with_graph(size_t n, Func func)
+    {
+        m_instance = static_cast<T*>(this);
+        m_instance->m_nodes.resize(n);
+        func();
+        m_instance->m_nodes.clear();
+        m_instance = nullptr;
+    }
+
+protected:
+    inline static thread_local T* m_instance = nullptr;
+};
+
+}