Add structural simplifier as optimization step for Yul.

test/libyul/yulOptimizerTests/structuralSimplifier/for_false_condition.sol

@@ -0,0 +1,10 @@
+{
+    for { let a := 42 } 0 { a := a } {
+        let b := a
+    }
+}
+// ----
+// structuralSimplifier
+// {
+//     let a := 42
+// }