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/when/whenOnNothing.txt

@@ -1,3 +1,6 @@
 package
 
-public fun foo(/*0*/ n: kotlin.Nothing, /*1*/ nn: kotlin.Nothing?): kotlin.Unit
+public fun test1(/*0*/ n: kotlin.Nothing): kotlin.Unit
+public fun test2(/*0*/ n: kotlin.Nothing?): kotlin.Unit
+public fun test3(/*0*/ n: kotlin.Nothing?): kotlin.Unit
+