Merge pull request #6848 from ethereum/invertibleRelation

Implement references using InvertibleRelation as data structure.

test/libyul/yulOptimizerTests/commonSubexpressionEliminator/clear_not_needed.yul

@@ -0,0 +1,21 @@
+{
+    let a := calldataload(0)
+    let x := calldataload(0x20)
+    x := a
+    let z := 0
+    x := z
+    a := 9
+    sstore(x, 3)
+}
+// ====
+// step: commonSubexpressionEliminator
+// ----
+// {
+//     let a := calldataload(0)
+//     let x := calldataload(0x20)
+//     x := a
+//     let z := 0
+//     x := z
+//     a := 9
+//     sstore(z, 3)
+// }