fix serialization of IR nop

bir/src/main/kotlin/gay/pizza/pork/bir/IrNop.kt

@@ -1,5 +1,8 @@
 package gay.pizza.pork.bir
 
+import kotlinx.serialization.Serializable
+
+@Serializable
 object IrNop : IrCodeElement() {
   override fun crawl(block: (IrElement) -> Unit) {}
 }