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/smartCasts/alwaysNull.fir.kt

@@ -3,7 +3,7 @@ fun foo(): String {
     s = null
     s?.length
     s<!UNSAFE_CALL!>.<!>length
-    if (s == null) return s!!
+    if (<!SENSELESS_COMPARISON!>s == null<!>) return s!!
     var t: String? = "y"
     if (t == null) t = "x"
     var x: Int? = null

compiler/testData/diagnostics/tests/smartCasts/castchecks/variables.fir.kt

@@ -15,7 +15,7 @@ object Impl : SomeSubClass {
 fun g(a: SomeClass?) {
     var b = (a as? SomeSubClass)?.foo
     b = "Hello"
-    if (b != null) {
+    if (<!SENSELESS_COMPARISON!>b != null<!>) {
         // 'a' cannot be cast to SomeSubClass!
         a<!UNSAFE_CALL!>.<!>hashCode()
         a.<!UNRESOLVED_REFERENCE!>foo<!>
@@ -24,7 +24,7 @@ fun g(a: SomeClass?) {
     }
     var c = a as? SomeSubClass
     c = Impl
-    if (c != null) {
+    if (<!SENSELESS_COMPARISON!>c != null<!>) {
         // 'a' cannot be cast to SomeSubClass
         a<!UNSAFE_CALL!>.<!>hashCode()
         a.<!UNRESOLVED_REFERENCE!>foo<!>

compiler/testData/diagnostics/tests/smartCasts/notNullorNotNull.fir.kt

@@ -1,6 +1,6 @@
 fun bar(x: Int?): Int {
     if (x != null) return -1
-    if (x == null) return -2
+    if (<!SENSELESS_COMPARISON!>x == null<!>) return -2
     // Should be unreachable
     return 2 + 2
-}
+}

compiler/testData/diagnostics/tests/smartCasts/ownerDeclaresBothModifies.fir.kt

@@ -7,7 +7,7 @@ fun foo(arg: Int?) {
         x.hashCode()
         x = null
     }
-    if (x != null) x = 42
+    if (<!SENSELESS_COMPARISON!>x != null<!>) x = 42
     // Unsafe because of lambda
     x<!UNSAFE_CALL!>.<!>hashCode()
 }

compiler/testData/diagnostics/tests/smartCasts/variables/whileWithBreak.fir.kt

@@ -4,7 +4,7 @@ fun String.next(): String {
 
 fun list(start: String) {
     var e: Any? = start
-    if (e==null) return
+    if (<!SENSELESS_COMPARISON!>e==null<!>) return
     while (e is String) {
         // Smart cast due to the loop condition
         if (e.length == 0)

compiler/testData/diagnostics/tests/smartCasts/varnotnull/inference.fir.kt

@@ -3,7 +3,7 @@ fun f() {
   var s: String?
   s = "a"
   var s1 = "" // String � ?
-  if (s != null) {    // Redundant
+  if (<!SENSELESS_COMPARISON!>s != null<!>) {    // Redundant
     s1.length
     // We can do smartcast here and below
     s1 = s.toString() // return String?
@@ -18,4 +18,4 @@ fun f() {
     val s3 = s.toString()
     s3.length
   }
-}
+}

compiler/testData/diagnostics/tests/smartCasts/varnotnull/nestedLoops.fir.kt

@@ -11,9 +11,9 @@ public fun foo(qq: String?): Int {
                 // p = r, r = q and q is not null
                 p.length
             } while (!x())
-        } while (r == null) // r = q and q is not null
+        } while (<!SENSELESS_COMPARISON!>r == null<!>) // r = q and q is not null
         if (!x()) break
     }
     // Smart cast is possible
     return q.length
-}
+}