[FIR] Adding a test case to show resolution of KT-29559 in K2

compiler/testData/diagnostics/tests/postfixIncSmartCast.fir.kt

@@ -0,0 +1,11 @@
+open class I {
+    operator fun inc(): ST = ST()
+}
+
+class ST : I()
+
+fun main() {
+    var local = I()
+    val x: ST = <!INITIALIZER_TYPE_MISMATCH!>local++<!>
+    val y: ST = local
+}