Add structural simplifier as optimization step for Yul.

libyul/optimiser/StructuralSimplifier.h

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+/*
+	This file is part of solidity.
+
+	solidity is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 3 of the License, or
+	(at your option) any later version.
+
+	solidity is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with solidity.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#pragma once
+
+#include <libyul/optimiser/ASTWalker.h>
+#include <libyul/optimiser/DataFlowAnalyzer.h>
+
+namespace yul
+{
+
+/**
+ * Structural simplifier. Performs the following simplification steps:
+ * - replace if with empty body with pop(condition)
+ * - replace if with true condition with its body
+ * - remove if with false condition
+ * - turn switch with single case into if
+ * - replace switch with only default case with pop(expression) and body
+ * - remove for with false condition
+ *
+ * Prerequisites: Disambiguator
+ *
+ * Important: Can only be used on EVM code.
+ */
+class StructuralSimplifier: public DataFlowAnalyzer
+{
+public:
+	using DataFlowAnalyzer::operator();
+	void operator()(Block& _block) override;
+private:
+	void simplify(std::vector<Statement>& _statements);
+	bool expressionAlwaysTrue(Expression const &_expression);
+	bool expressionAlwaysFalse(Expression const &_expression);
+};
+
+}