Add structural simplifier as optimization step for Yul.

libyul/optimiser/Utilities.h

@@ -20,6 +20,7 @@
 
 #pragma once
 
+#include <libdevcore/Common.h>
 #include <libyul/AsmDataForward.h>
 
 namespace yul
@@ -28,4 +29,6 @@ namespace yul
 /// Removes statements that are just empty blocks (non-recursive).
 void removeEmptyBlocks(Block& _block);
 
+dev::u256 valueOfNumberLiteral(Literal const& _literal);
+
 }