FIR checker: SENSELESS_(COMPARISON|NULL_IN_WHEN)

Currently DFA does not set "definitely equal to null" for access to variables that got assigned `null`. For example, FIR should mark the following line as SENSELESS_COMPARISON due to `s = null` above.
 https://github.com/JetBrains/kotlin/blob/d1531f9cdd5852352c0133198706125dc63b6007/compiler/testData/diagnostics/tests/smartCasts/alwaysNull.fir.kt#L6

The problem is at https://github.com/JetBrains/kotlin/blob/7e9f27436a77de1c76e3705da7aa1fbe8938336b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/dfa/FirDataFlowAnalyzer.kt#L1104

For null assignment, ideally the type should be `Nothing?`. This is
addressed in a followup commit instead.

compiler/tests-spec/testData/diagnostics/notLinked/dfa/neg/7.fir.kt

@@ -28,7 +28,7 @@ fun case_3(x: Boolean?) {
 
 // TESTCASE NUMBER: 4
 fun case_4(x: Boolean?) {
-    if (if (x != null) x else x != null) {
+    if (if (x != null) x else <!SENSELESS_COMPARISON!>x != null<!>) {
         <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Boolean?")!>x<!>
         <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Boolean?")!>x<!><!UNSAFE_CALL!>.<!>not()
     }