Test refactoring

compiler/testData/diagnostics/tests/smartCasts/kt6819.kt

@@ -0,0 +1,7 @@
+public fun test(o: String?): Boolean {
+    return when {
+        // Data flow info should propagate from o == null to o.length()
+        o == null, <!DEBUG_INFO_SMARTCAST!>o<!>.length() == 0 -> false 
+        else -> true
+    }
+}