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/testsWithStdLib/contracts/dsl/errors/illegalEqualsCondition.fir.kt

@@ -18,7 +18,7 @@ fun identityEqualsWithVariables(x: Any?, y: Any?) {
 
 fun equalConstants() {
     contract {
-        <!ERROR_IN_CONTRACT_DESCRIPTION!>returns() implies (null == null)<!>
+        <!ERROR_IN_CONTRACT_DESCRIPTION!>returns() implies (<!SENSELESS_COMPARISON!>null == null<!>)<!>
     }
 }
 
@@ -27,4 +27,4 @@ fun equalNullWithCall() {
     contract {
         <!ERROR_IN_CONTRACT_DESCRIPTION!>returns() implies (get() == null)<!>
     }
-}
+}

compiler/testData/diagnostics/testsWithStdLib/contracts/dsl/errors/unlabeledReceiver.fir.kt

@@ -6,7 +6,7 @@ import kotlin.contracts.*
 
 fun Any?.foo(): Boolean {
     contract {
-        <!ERROR_IN_CONTRACT_DESCRIPTION!>returns(true) implies (this != null)<!>
+        <!ERROR_IN_CONTRACT_DESCRIPTION!>returns(true) implies (<!SENSELESS_COMPARISON!>this != null<!>)<!>
     }
     return this != null
-}
+}