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/alwaysNull.kt

@@ -4,7 +4,7 @@ fun foo(): String {
     s = null
     <!DEBUG_INFO_CONSTANT!>s<!>?.length
     <!OI;DEBUG_INFO_CONSTANT!>s<!><!UNSAFE_CALL!>.<!>length
-    if (<!SENSELESS_COMPARISON!><!DEBUG_INFO_CONSTANT!>s<!> == null<!>) <!NI;UNREACHABLE_CODE!>return<!> <!ALWAYS_NULL!>s<!>!!
+    if (<!SENSELESS_COMPARISON!><!DEBUG_INFO_CONSTANT!>s<!> == null<!>) return <!ALWAYS_NULL!>s<!>!!
     var t: String? = "y"
     if (t == null) t = "x"
     var x: Int? = null

compiler/testData/diagnostics/tests/smartCasts/publicVals/delegate.kt

@@ -14,7 +14,7 @@ class Example {
 
     public fun foo(): String {
         // Smart cast is not possible if property is delegated
-        return <!NI;TYPE_MISMATCH!>if (p != null) <!NI;SMARTCAST_IMPOSSIBLE, SMARTCAST_IMPOSSIBLE!>p<!> else ""<!>
+        return if (p != null) <!SMARTCAST_IMPOSSIBLE!>p<!> else ""
     }
 
     public fun bar(): String {

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<!>
 }
 

compiler/testData/diagnostics/tests/smartCasts/varnotnull/postfixNotnullClassIncrement.kt

@@ -1,7 +1,7 @@
 // !WITH_NEW_INFERENCE
 class MyClass
 
-operator fun MyClass.inc(): MyClass { <!UNREACHABLE_CODE!>return<!> null!! }
+operator fun MyClass.inc(): MyClass { <!OI;UNREACHABLE_CODE!>return<!> null!! }
 
 public fun box() : MyClass? {
     var i : MyClass?

compiler/testData/diagnostics/tests/smartCasts/varnotnull/prefixNotnullClassIncrement.kt

@@ -1,7 +1,7 @@
 // !WITH_NEW_INFERENCE
 class MyClass
 
-operator fun MyClass.inc(): MyClass { <!UNREACHABLE_CODE!>return<!> null!! }
+operator fun MyClass.inc(): MyClass { <!OI;UNREACHABLE_CODE!>return<!> null!! }
 
 public fun box() {
     var i : MyClass?