JSSpecCompiler: Add bare-bones DCE pass

Right now the only dead code it eliminates is the unused phi nodes.

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

@@ -0,0 +1,86 @@
+/*
+ * Copyright (c) 2023, Dan Klishch <danilklishch@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Vector.h>
+
+#include "Compiler/EnableGraphPointers.h"
+
+namespace JSSpecCompiler {
+
+namespace Detail {
+template<typename GraphVertex, typename GraphNode>
+class StronglyConnectedComponents
+    : private EnableGraphPointers<StronglyConnectedComponents<GraphVertex, GraphNode>> {
+    using Self = StronglyConnectedComponents<GraphVertex, GraphNode>;
+    using Vertex = typename EnableGraphPointers<Self>::Vertex;
+
+public:
+    StronglyConnectedComponents(Vector<GraphNode> const& graph)
+        : m_graph(graph)
+    {
+    }
+
+    Vector<Vector<GraphVertex>> find()
+    {
+        Vector<Vector<GraphVertex>> result;
+        size_t n = m_graph.size();
+        Self::with_graph(n, [&] {
+            for (size_t i = 0; i < m_graph.size(); ++i)
+                find_order(i);
+            for (size_t i = n; i--;) {
+                if (!m_order[i]->is_processed) {
+                    result.empend();
+                    find_component(GraphVertex(m_order[i]), result.last());
+                }
+            }
+        });
+        return result;
+    }
+
+private:
+    friend EnableGraphPointers<Self>;
+
+    void find_order(Vertex u)
+    {
+        if (u->is_visited)
+            return;
+        u->is_visited = true;
+
+        for (auto v : GraphVertex(u)->incoming_edges)
+            find_order(Vertex(v));
+        m_order.append(u);
+    }
+
+    void find_component(GraphVertex u, Vector<GraphVertex>& current_scc)
+    {
+        current_scc.empend(u);
+        Vertex(u)->is_processed = true;
+        for (auto v : u->outgoing_edges)
+            if (!Vertex(v)->is_processed)
+                find_component(v, current_scc);
+    }
+
+    struct NodeData {
+        bool is_visited = false;
+        bool is_processed = false;
+    };
+
+    Vector<GraphNode> const& m_graph;
+    Vector<NodeData> m_nodes;
+    Vector<Vertex> m_order;
+};
+}
+
+template<typename NodeData>
+auto find_strongly_connected_components(Vector<NodeData> const& graph)
+{
+    using Vertex = RemoveCVReference<decltype(graph[0].outgoing_edges[0])>;
+    return Detail::StronglyConnectedComponents<Vertex, NodeData>(graph).find();
+}
+
+}