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/testData/diagnostics/tests/dataFlowInfoTraversal/IfThenElse.fir.kt

@@ -26,5 +26,5 @@ fun foo() {
     if (z != null) bar(z)
     bar(<!ARGUMENT_TYPE_MISMATCH!>z<!>)
     bar(z!!)
-    if (z != null) bar(z<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>)
+    if (<!SENSELESS_COMPARISON!>z != null<!>) bar(z<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>)
 }