FIR DFA: smartcast variable to Nothing? on null assignment

In order to make resolution still work for members not available from
`Nothing`, we track the type without `Nothing?` and use that for
resolution instead.

compiler/tests-spec/testData/diagnostics/notLinked/dfa/neg/29.fir.kt

@@ -9,8 +9,8 @@ fun case_1(a: Any?) {
         if (true) continue
     }
 
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any? & kotlin.Any?")!>a<!>
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any? & kotlin.Any?")!>a<!><!UNSAFE_CALL!>.<!>equals(10)
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any? & kotlin.Nothing?")!>a<!>
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any? & kotlin.Nothing?")!>a<!><!UNSAFE_CALL!>.<!>equals(10)
 }
 
 // TESTCASE NUMBER: 2