Relax rules about inferring to Nothing for special calls

#KT-37388 Fixed
 #KT-38427 Fixed
 #KT-39953 Fixed
 #KT-38899 Fixed

compiler/testData/diagnostics/tests/infos/SmartCasts.kt

@@ -142,10 +142,10 @@ fun getStringLength(obj : Any) : Char? {
 }
 
 fun toInt(i: Int?): Int = if (i != null) <!DEBUG_INFO_SMARTCAST!>i<!> else 0
-fun illegalWhenBody(a: Any): Int = <!NI;TYPE_MISMATCH!><!NO_ELSE_IN_WHEN!>when<!>(a) {
+fun illegalWhenBody(a: Any): Int = <!NO_ELSE_IN_WHEN!>when<!>(a) {
     is Int -> <!DEBUG_INFO_SMARTCAST!>a<!>
-    is String -> <!OI;TYPE_MISMATCH!>a<!>
-}<!>
+    is String -> <!TYPE_MISMATCH!>a<!>
+}
 fun illegalWhenBlock(a: Any): Int {
     when(a) {
         is Int -> return <!DEBUG_INFO_SMARTCAST!>a<!>