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/codegen/box/smartCasts/nullSmartCast.kt

@@ -1,6 +1,5 @@
 // DONT_TARGET_EXACT_BACKEND: WASM
 // WASM_MUTE_REASON: NULL_REF_CAST
-// IGNORE_BACKEND_FIR: JVM_IR
 fun String?.foo() = this ?: "OK"
 
 fun foo(i: Int?): String {