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/varnotnull/varIntNull.fir.kt

@@ -1,5 +1,5 @@
 fun foo(): Int {
     var i: Int? = 42
     i = null
-    return i <!UNSAFE_OPERATOR_CALL!>+<!> 1
+    return <!RETURN_TYPE_MISMATCH, TYPE_MISMATCH!>i + 1<!>
 }