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/testData/diagnostics/tests/smartCasts/alwaysNull.fir.kt

@@ -7,7 +7,7 @@ fun foo(): String {
     var t: String? = "y"
     if (t == null) t = "x"
     var x: Int? = null
-    if (x == null) x <!UNSAFE_OPERATOR_CALL!>+=<!> null
+    if (x == null) <!ASSIGNMENT_TYPE_MISMATCH!>x += null<!>
     return t + s
 }
 
@@ -16,7 +16,7 @@ fun String?.gav() {}
 fun bar(s: String?) {
     if (s != null) return
     s.gav()
-    s <!USELESS_CAST!>as? String<!>
-    s <!USELESS_CAST!>as String?<!>
+    s as? String
+    s as String?
     s as String
 }