Add tests.

test/libyul/objectCompiler/simple_optimizer.yul

@@ -0,0 +1,23 @@
+// optimize
+{
+  let x := calldataload(0)
+  let y := calldataload(0)
+  let z := sub(y, x)
+  sstore(add(x, 0), z)
+}
+// ----
+// Assembly:
+//     /* "source":38:39   */
+//   0x00
+//     /* "source":109:110   */
+//   dup1
+//     /* "source":38:39   */
+//   dup2
+//     /* "source":25:40   */
+//   calldataload
+//     /* "source":91:111   */
+//   sstore
+//     /* "source":12:113   */
+//   pop
+// Bytecode: 60008081355550
+// Opcodes: PUSH1 0x0 DUP1 DUP2 CALLDATALOAD SSTORE POP