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/fir/analysis-tests/testData/resolve/extendedCheckers/CanBeValChecker.kt

@@ -137,7 +137,7 @@ fun main(args: Array<String?>) {
         <!ASSIGNED_VALUE_IS_NEVER_READ!>a<!> = args[0]
     } else {
         a  = args.toString()
-        if (a != null && a.equals("cde")) return
+        if (<!SENSELESS_COMPARISON!>a != null<!> && a.equals("cde")) return
     }
 }
 

compiler/fir/analysis-tests/testData/resolve/smartcasts/boundSmartcasts/boundSmartcastsInBranches.kt

@@ -107,7 +107,7 @@ fun test_7() {
         y<!UNSAFE_CALL!>.<!>length // Bad
         z.length // OK
     }
-    if (y != null) {
+    if (<!SENSELESS_COMPARISON!>y != null<!>) {
         x<!UNSAFE_CALL!>.<!>length // Bad
         y.length // OK
         z<!UNSAFE_CALL!>.<!>length // Bad

compiler/fir/analysis-tests/testData/resolveWithStdlib/contracts/fromSource/good/returnsImplies/propertyAccessors.kt

@@ -14,7 +14,7 @@ var Any?.isNotNull: Boolean
     set(value) {
         contract {
             returns() implies (this@isNotNull != null)
-            <!ERROR_IN_CONTRACT_DESCRIPTION!>require(this != null)<!>
+            <!ERROR_IN_CONTRACT_DESCRIPTION!>require(<!SENSELESS_COMPARISON!>this != null<!>)<!>
         }
     }