Fix behaviour of NI in testData

After one of previous commits (my or Mikhail Zarechenskiy) that test
  was broken and it's behaviour was not fixed. That test's behaviour
  is strange (why at one case we have `UNREACHABLE_CODE` and haven't
  at other) and needs further investigation

compiler/testData/diagnostics/tests/nullableTypes/nullAssertOnTypeWithNullableUpperBound.kt

@@ -3,7 +3,7 @@ fun <T> test(t: T): T {
     if (t != null) {
         return t<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
     }
-    <!NI;UNREACHABLE_CODE!>return<!> <!ALWAYS_NULL!>t<!>!!
+    return <!ALWAYS_NULL!>t<!>!!
 }
 
 fun <T> T.testThis(): String {