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/pos/37.fir.kt

@@ -98,7 +98,7 @@ fun case_9(x: Boolean?) {
     while (x ?: return)
     while (x == null)
 
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Boolean? & kotlin.Boolean")!>x<!>
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Boolean? & kotlin.Nothing")!>x<!>
     <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Boolean? & kotlin.Boolean")!>x<!>.equals(10)
 }