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/smartCasts/variables/ifNullAssignment.kt

@@ -3,7 +3,7 @@
 
 fun basic(): String {
     var current: String? = null
-    current = if (current == null) "bar" else <!NI;DEBUG_INFO_SMARTCAST!>current<!>
+    current = if (current == null) "bar" else current
     return <!DEBUG_INFO_SMARTCAST!>current<!>
 }