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/dataFlowInfoTraversal/DeepIf.fir.kt

@@ -6,24 +6,24 @@ fun foo() {
 
     if (x != null) {
         bar(x)
-        if (x != null) {
+        if (<!SENSELESS_COMPARISON!>x != null<!>) {
             bar(x)
             if (1 < 2) bar(x)
             if (1 > 2) bar(x)
         }
-        if (x == null) {
+        if (<!SENSELESS_COMPARISON!>x == null<!>) {
             bar(x)
         }
-        if (x == null) bar(x) else bar(x)
+        if (<!SENSELESS_COMPARISON!>x == null<!>) bar(x) else bar(x)
         bar(bar(x))
-    } else if (x == null) {
+    } else if (<!SENSELESS_COMPARISON!>x == null<!>) {
         bar(<!ARGUMENT_TYPE_MISMATCH!>x<!>)
-        if (x != null) {
+        if (<!SENSELESS_COMPARISON!>x != null<!>) {
             bar(x)
-            if (x == null) bar(x)
-            if (x == null) bar(x) else bar(x)
+            if (<!SENSELESS_COMPARISON!>x == null<!>) bar(x)
+            if (<!SENSELESS_COMPARISON!>x == null<!>) bar(x) else bar(x)
             bar(bar(x) + bar(x))
-        } else if (x == null) {
+        } else if (<!SENSELESS_COMPARISON!>x == null<!>) {
             bar(<!ARGUMENT_TYPE_MISMATCH!>x<!>)
         }
     }

compiler/testData/diagnostics/tests/dataFlowInfoTraversal/For.fir.kt

@@ -10,7 +10,7 @@ fun foo() {
         bar(x)
         for (q in a) {
             bar(x)
-            if (x == null) bar(x)
+            if (<!SENSELESS_COMPARISON!>x == null<!>) bar(x)
         }
     }
 

compiler/testData/diagnostics/tests/dataFlowInfoTraversal/IfThenElse.fir.kt

@@ -26,5 +26,5 @@ fun foo() {
     if (z != null) bar(z)
     bar(<!ARGUMENT_TYPE_MISMATCH!>z<!>)
     bar(z!!)
-    if (z != null) bar(z<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>)
+    if (<!SENSELESS_COMPARISON!>z != null<!>) bar(z<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>)
 }

compiler/testData/diagnostics/tests/dataFlowInfoTraversal/IfThenElseBothInvalid.fir.kt

@@ -9,7 +9,7 @@ fun foo() {
         2<!OVERLOAD_RESOLUTION_AMBIGUITY!>+<!><!SYNTAX!><!>
     }
     else {
-        if (x == null) return
+        if (<!SENSELESS_COMPARISON!>x == null<!>) return
         2<!OVERLOAD_RESOLUTION_AMBIGUITY!>+<!><!SYNTAX!><!>
     }
     bar(x)

compiler/testData/diagnostics/tests/dataFlowInfoTraversal/Return.fir.kt

@@ -8,10 +8,10 @@ fun foo(): Int {
     if (x != null) return x
 
     val y: Int? = null
-    if (y == null) return <!RETURN_TYPE_MISMATCH!>if (y != null) y else y<!>
+    if (y == null) return <!RETURN_TYPE_MISMATCH!>if (<!SENSELESS_COMPARISON!>y != null<!>) y else y<!>
 
     val z: Int? = null
-    if (z != null) return if (z == null) z else z
+    if (z != null) return if (<!SENSELESS_COMPARISON!>z == null<!>) z else z
 
     return <!RETURN_TYPE_MISMATCH!>z<!>
 }

compiler/testData/diagnostics/tests/dataFlowInfoTraversal/When.fir.kt

@@ -11,7 +11,7 @@ fun foo() {
     }
 
     when (x) {
-        0 -> { if (x == null) return }
+        0 -> { if (<!SENSELESS_COMPARISON!>x == null<!>) return }
         else -> { if (x == null) return }
     }
     bar(x)