[FIR2IR] Fix generation of val initialization after smart-cast

If neither setter nor backing field were found for an assignment call,
search overridden properties for backing fields.

^KT-57105 Fixed

compiler/testData/ir/irText/classes/smartCastInValInitialization.fir.kt.txt

@@ -19,7 +19,7 @@ open class MessageBusImpl {
 
   init {
     <this> as RootBus /*~> Unit */
-    error("") /* ErrorCallExpression */
+    <this> /*as RootBus */.#parentBus = null
   }
 
 }