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

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

@@ -0,0 +1,23 @@
+// !WITH_NEW_INFERENCE
+fun foo(): String {
+    var s: String?
+    s = null
+    s?.<!UNRESOLVED_REFERENCE!>length<!>
+    s.<!UNRESOLVED_REFERENCE!>length<!>
+    if (s == null) return s!!
+    var t: String? = "y"
+    if (t == null) t = "x"
+    var x: Int? = null
+    if (x == null) <!UNRESOLVED_REFERENCE!>x += null<!>
+    return t + s
+}
+
+fun String?.gav() {}
+
+fun bar(s: String?) {
+    if (s != null) return
+    s.gav()
+    s as? String
+    s as String?
+    s as String
+}