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/smartcasts/nullabilitySmartcastWhenNullability.fir.kt

@@ -39,7 +39,7 @@ fun notNullWhenNotNull (x: Int?): Int? {
 
 fun testNotNullWhenNotNull (x: Int?) {
     if (notNullWhenNotNull(x) == null) {
-        x == null
+        <!SENSELESS_COMPARISON!>x == null<!>
     }
     else {
         x<!UNSAFE_CALL!>.<!>dec()
@@ -49,7 +49,7 @@ fun testNotNullWhenNotNull (x: Int?) {
         x<!UNSAFE_CALL!>.<!>dec()
     }
     else {
-        x == null
+        <!SENSELESS_COMPARISON!>x == null<!>
     }
 
     x<!UNSAFE_CALL!>.<!>dec()

compiler/testData/diagnostics/testsWithStdLib/contracts/smartcasts/throwsEffect.fir.kt

@@ -60,7 +60,7 @@ fun testAtLeastOnceFlow(x: Any?) {
     do {
         myAssert(x is String)
         x.length
-    } while (x != null)
+    } while (<!SENSELESS_COMPARISON!>x != null<!>)
 
     x.length
 }

compiler/testData/diagnostics/testsWithStdLib/contracts/smartcasts/when/kt36818.fir.kt

@@ -5,7 +5,7 @@ fun main(x1: Double?, range: ClosedRange<Double>) {
     }
 
     when {
-        x1 == null -> throw Exception()
+        <!SENSELESS_COMPARISON!>x1 == null<!> -> throw Exception()
         x1 in range -> {}
     }
 }