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/generics/nullability/smartCasts.fir.kt

@@ -9,7 +9,7 @@ fun CharSequence.bar4() {}
 fun <T : CharSequence?> foo(x: T) {
 
     if (x != null) {
-        if (x != null) {}
+        if (<!SENSELESS_COMPARISON!>x != null<!>) {}
 
         x.length
         x<!UNNECESSARY_SAFE_CALL!>?.<!>length

compiler/testData/diagnostics/tests/generics/nullability/smartCastsOnThis.fir.kt

@@ -8,7 +8,7 @@ fun CharSequence.bar4() {}
 
 fun <T : String?> T.foo() {
     if (this != null) {
-        if (this != null) {}
+        if (<!SENSELESS_COMPARISON!>this != null<!>) {}
 
         length
         this<!UNNECESSARY_SAFE_CALL!>?.<!>length

compiler/testData/diagnostics/tests/generics/nullability/smartCastsValueArgument.fir.kt

@@ -8,7 +8,7 @@ fun <T : CharSequence?> foo(x: T) {
     var y1: CharSequence = ""
     var y2: String = ""
     if (x != null) {
-        if (x != null) {}
+        if (<!SENSELESS_COMPARISON!>x != null<!>) {}
 
         y1 = x
         y2 = <!ASSIGNMENT_TYPE_MISMATCH!>x<!>