[FIR-TEST] Add new testdata generated after changes in previous commit

compiler/testData/diagnostics/tests/smartCasts/incDecToNull.fir.kt

@@ -0,0 +1,10 @@
+class IncDec {
+    operator fun inc(): Unit {}
+}
+
+fun foo(): IncDec {
+    var x = IncDec()
+    x = x++
+    x++
+    return x
+}