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

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

@@ -0,0 +1,13 @@
+fun baz(s: String?, u: String?): String {
+    val t = when(s) {
+        is String -> {
+            if (u == null) return s
+            u
+        }
+        else -> {
+            if (u == null) return ""
+            u
+        }
+    }
+    return t
+}