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

compiler/testData/diagnostics/tests/smartCasts/loops/whileInCondition.fir.kt

@@ -0,0 +1,6 @@
+fun foo(s: String?): Int {
+    while (s!!.length > 0) {
+        s.length
+    }
+    return s.length
+}