Fix expression simplifying by moving from SSAValueTracker to DataFlowAnalyzer as a base.

test/libyul/yulOptimizerTests/expressionSimplifier/reassign.yul

@@ -0,0 +1,12 @@
+{
+	let x := mload(0)
+	x := 0
+	mstore(0, add(7, x))
+}
+// ----
+// expressionSimplifier
+// {
+//     let x := mload(0)
+//     x := 0
+//     mstore(0, 7)
+// }

test/libyul/yulOptimizerTests/fullSuite/reuse_vars_bug_in_simplifier.yul

@@ -0,0 +1,16 @@
+{
+	// This tests that a bug is fixed that was related to just taking
+	// values of SSA variables in account and not clearing them when
+	// a dependency was reassigned.
+	pop(foo())
+	function foo() -> x_9
+	{
+		x_9 := sub(1,sub(x_9,1))
+		mstore(sub(1,div(sub(x_9,1),sub(1,sub(x_9,1)))), 1)
+	}
+}
+// ----
+// fullSuite
+// {
+//     mstore(1, 1)
+// }