More accurate handling of "always null" for receivers

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

@@ -4,7 +4,7 @@ fun foo() {
     v = "abc"
     <!DEBUG_INFO_SMARTCAST!>v<!>.length
     v = null
-    <!ALWAYS_NULL!>v<!><!UNSAFE_CALL!>.<!>length
+    <!DEBUG_INFO_CONSTANT!>v<!><!UNSAFE_CALL!>.<!>length
     v = "abc"
     <!DEBUG_INFO_SMARTCAST!>v<!>.length
 }

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

@@ -3,5 +3,5 @@ fun foo() {
     // It is possible in principle to provide smart cast here
     v<!UNSAFE_CALL!>.<!>length
     v = null
-    <!ALWAYS_NULL!>v<!><!UNSAFE_CALL!>.<!>length
+    <!DEBUG_INFO_CONSTANT!>v<!><!UNSAFE_CALL!>.<!>length
 }

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

@@ -1,5 +1,5 @@
 fun foo(): Int {
     var s: String? = <!VARIABLE_WITH_REDUNDANT_INITIALIZER!>"abc"<!>
     s = null
-    return <!ALWAYS_NULL!>s<!><!UNSAFE_CALL!>.<!>length
+    return <!DEBUG_INFO_CONSTANT!>s<!><!UNSAFE_CALL!>.<!>length
 }