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/Passes/DeadCodeEliminationPass.h

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2023, Dan Klishch <danilklishch@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include "Compiler/EnableGraphPointers.h"
+#include "Compiler/GenericASTPass.h"
+#include "Compiler/StronglyConnectedComponents.h"
+
+namespace JSSpecCompiler {
+
+class DeadCodeEliminationPass
+    : public IntraproceduralCompilerPass
+    , private RecursiveASTVisitor
+    , private EnableGraphPointers<DeadCodeEliminationPass, SSAVariableDeclarationRef> {
+public:
+    inline static constexpr StringView name = "dce"sv;
+
+    using IntraproceduralCompilerPass::IntraproceduralCompilerPass;
+
+protected:
+    void process_function() override;
+
+private:
+    friend EnableGraphPointers;
+
+    static Vertex as_vertex(Variable* variable);
+    RecursionDecision on_entry(Tree tree) override;
+    void on_leave(Tree tree) override;
+    void remove_unused_phi_nodes();
+
+    struct NodeData {
+        Vector<Vertex> outgoing_edges;
+        Vector<Vertex> incoming_edges;
+
+        bool is_referenced = false;
+    };
+
+    Vector<NodeData> m_nodes;
+};
+
+}