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/tests-spec/testData/diagnostics/notLinked/objects/inheritance/neg/1.fir.kt

@@ -13,7 +13,7 @@ open class Foo(val prop: Int) {
 }
 
 fun box(): String? {
-    if (Foo(42) == null) return null
+    if (<!SENSELESS_COMPARISON!>Foo(42) == null<!>) return null
 
     return "OK"
 }

compiler/tests-spec/testData/diagnostics/notLinked/objects/inheritance/neg/2.fir.kt

@@ -13,7 +13,7 @@ open class Foo(val prop: Int) {
 }
 
 fun box(): String? {
-    if (Foo(42) == null) return null
+    if (<!SENSELESS_COMPARISON!>Foo(42) == null<!>) return null
 
     return "OK"
 }

compiler/tests-spec/testData/diagnostics/notLinked/objects/inheritance/neg/3.fir.kt

@@ -13,7 +13,7 @@ open class Foo(val prop: Int) {
 }
 
 fun box(): String? {
-    if (Foo.MyObject == null) return null
+    if (<!SENSELESS_COMPARISON!>Foo.MyObject == null<!>) return null
 
     return "OK"
 }

compiler/tests-spec/testData/diagnostics/notLinked/objects/inheritance/neg/4.fir.kt

@@ -13,7 +13,7 @@ open class Foo(val prop: Int) {
 }
 
 fun box(): String? {
-    if (Foo(42) == null) return null
+    if (<!SENSELESS_COMPARISON!>Foo(42) == null<!>) return null
 
     return "OK"
 }

compiler/tests-spec/testData/diagnostics/notLinked/objects/inheritance/neg/5.fir.kt

@@ -13,7 +13,7 @@ open class Foo(val prop: Int) {
 }
 
 fun box(): String? {
-    if (Foo(42) == null) return null
+    if (<!SENSELESS_COMPARISON!>Foo(42) == null<!>) return null
 
     return "OK"
 }

compiler/tests-spec/testData/diagnostics/notLinked/objects/inheritance/neg/6.fir.kt

@@ -13,7 +13,7 @@ open class Foo(val prop: Int) {
 }
 
 fun box(): String? {
-    if (Foo(42) == null) return null
+    if (<!SENSELESS_COMPARISON!>Foo(42) == null<!>) return null
 
     return "OK"
 }

compiler/tests-spec/testData/diagnostics/notLinked/objects/inheritance/neg/7.fir.kt

@@ -13,7 +13,7 @@ open class Foo(val prop: Int) {
 }
 
 fun box(): String? {
-    if (Foo.MyObject == null) return null
+    if (<!SENSELESS_COMPARISON!>Foo.MyObject == null<!>) return null
 
     return "OK"
 }