[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/codegen/box/smartCasts/propertyInitializationAfterSmartCast.kt

@@ -0,0 +1,16 @@
+// ISSUE: KT-57105
+
+class RootBus: MessageBusImpl()
+
+open class MessageBusImpl {
+    val parentBus: Any?
+
+    init {
+        this as RootBus
+        parentBus = "OK"
+    }
+}
+
+fun box(): String {
+    return RootBus().parentBus as String
+}