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/castchecks/variables.fir.kt

@@ -15,7 +15,7 @@ object Impl : SomeSubClass {
 fun g(a: SomeClass?) {
     var b = (a as? SomeSubClass)?.foo
     b = "Hello"
-    if (b != null) {
+    if (<!SENSELESS_COMPARISON!>b != null<!>) {
         // 'a' cannot be cast to SomeSubClass!
         a<!UNSAFE_CALL!>.<!>hashCode()
         a.<!UNRESOLVED_REFERENCE!>foo<!>
@@ -24,7 +24,7 @@ fun g(a: SomeClass?) {
     }
     var c = a as? SomeSubClass
     c = Impl
-    if (c != null) {
+    if (<!SENSELESS_COMPARISON!>c != null<!>) {
         // 'a' cannot be cast to SomeSubClass
         a<!UNSAFE_CALL!>.<!>hashCode()
         a.<!UNRESOLVED_REFERENCE!>foo<!>