Always null detection

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

@@ -0,0 +1,11 @@
+fun foo(): String {
+    var s: String?
+    s = null
+    <!ALWAYS_NULL!>s<!>?.length
+    if (<!ALWAYS_NULL!>s<!> == null) s = "z"
+    var t: String? = "y"
+    if (t == null) t = "x"
+    var x: Int? = null
+    if (x == null) <!TYPE_MISMATCH!><!DEBUG_INFO_SMARTCAST!>x<!> += null<!>
+    return <!DEBUG_INFO_SMARTCAST!>t<!> + s
+}