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

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

@@ -0,0 +1,10 @@
+fun baz(s: String?): Int {
+    return if (s == null) {
+        ""
+    }
+    else {
+        val u: String? = null
+        if (u == null) return 0
+        u
+    }.length
+}