JSSpecCompiler: Add control flow graph simplification pass

It removes empty `BasicBlock`s with an unconditional jump continuation
and then removes unreferenced blocks from the graph.

Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Compiler/Passes/CFGSimplificationPass.h

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2023, Dan Klishch <danilklishch@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include "Compiler/CompilerPass.h"
+#include "Compiler/ControlFlowGraph.h"
+
+namespace JSSpecCompiler {
+
+// CFGSimplificationPass removes empty `BasicBlock`s with an unconditional jump continuation. It
+// also removes unreferenced blocks from the graph.
+class CFGSimplificationPass : public IntraproceduralCompilerPass {
+public:
+    inline static constexpr StringView name = "cfg-simplification"sv;
+
+    using IntraproceduralCompilerPass::IntraproceduralCompilerPass;
+
+protected:
+    void process_function() override;
+
+private:
+    enum class State : char {
+        NotUsed,
+        CurrentlyInside,
+        Used,
+    };
+
+    bool compute_replacement_block(size_t i);
+    void compute_referenced_blocks(BasicBlockRef block);
+
+    Vector<BasicBlockRef> m_replacement;
+    Vector<State> m_state;
+};
+
+}