Also substitute variables in CSE.

test/libjulia/yulOptimizerTests/commonSubexpressionEliminator/variable_for_variable.yul

@@ -0,0 +1,27 @@
+{
+
+    let a := mload(0)
+    let b := add(a, 7)
+    let c := a
+    let d := c
+    let x := add(a, b)
+    // CSE has to recognize equality with x here.
+    let y := add(d, add(c, 7))
+    // some reassignments
+    b := mload(a)
+    a := b
+    mstore(2, a)
+}
+// ----
+// commonSubexpressionEliminator
+// {
+//     let a := mload(0)
+//     let b := add(a, 7)
+//     let c := a
+//     let d := a
+//     let x := add(a, b)
+//     let y := x
+//     b := mload(a)
+//     a := b
+//     mstore(2, b)
+// }