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/smartCasts/varnotnull/inference.fir.kt

@@ -3,7 +3,7 @@ fun f() {
   var s: String?
   s = "a"
   var s1 = "" // String � ?
-  if (s != null) {    // Redundant
+  if (<!SENSELESS_COMPARISON!>s != null<!>) {    // Redundant
     s1.length
     // We can do smartcast here and below
     s1 = s.toString() // return String?
@@ -18,4 +18,4 @@ fun f() {
     val s3 = s.toString()
     s3.length
   }
-}
+}