Also optimize memory.

test/libyul/yulOptimizerTests/loadResolver/second_store_same_value.yul

@@ -0,0 +1,19 @@
+{
+    let x := calldataload(1)
+    sstore(x, 7)
+    sstore(calldataload(0), 7)
+    // We can replace this because both values that were
+    // written are 7.
+    mstore(0, sload(x))
+}
+// ====
+// step: loadResolver
+// ----
+// {
+//     let x := calldataload(1)
+//     let _2 := 7
+//     sstore(x, _2)
+//     let _4 := 0
+//     sstore(calldataload(_4), _2)
+//     mstore(_4, _2)
+// }