Add test on smartcast from while-assigned var

Currently behavior is undesired, it will be fixed in the next commit
(see KT-27084)

compiler/testData/diagnostics/tests/smartCasts/loops/whileNullAssignToSomething.kt

@@ -0,0 +1,9 @@
+fun main(args: Array<String>) {
+    var result: String? = null
+    var i = 0
+    while (result == null) {
+        if (i == 10) result = "non null"
+        else i++
+    }
+    result<!UNSAFE_CALL!>.<!>length
+}