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/nullabilityAndSmartCasts/SenselessNullInWhen.fir.kt

@@ -1,10 +0,0 @@
-//KT-2457 Verify error when comparing not null value with null in when
-
-package kt2457
-
-fun foo(i: Int) : Int =
-    when (i) {
-        1 -> 1
-        null -> 1
-        else -> 1
-    }

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/SenselessNullInWhen.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 //KT-2457 Verify error when comparing not null value with null in when
 
 package kt2457

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/kt1680.fir.kt

@@ -1,14 +0,0 @@
-//KT-1680 Warn if non-null variable is compared to null
-package kt1680
-
-fun foo() {
-    val x = 1
-    if (x != null) {} // <-- need a warning here!
-    if (x == null) {}
-    if (null != x) {}
-    if (null == x) {}
-
-    val y : Int? = 1
-    if (y != null) {}
-    if (y == null) {}
-}

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/kt1680.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 //KT-1680 Warn if non-null variable is compared to null
 package kt1680
 

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/kt2109.fir.kt

@@ -13,7 +13,7 @@ fun A?.bar() {
 }
 
 fun A.baz() {
-    if (this == null) {
+    if (<!SENSELESS_COMPARISON!>this == null<!>) {
         return
     }
     foo()

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/kt2223.fir.kt

@@ -1,8 +0,0 @@
-//KT-2223 Comparing non-null value with null might produce helpful warning
-package kt2223
-
-fun foo() {
-    val x: Int? = null
-    if (x == null) return
-    if (x == null) return
-}

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/kt2223.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 //KT-2223 Comparing non-null value with null might produce helpful warning
 package kt2223
 

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/senslessComparisonWithNullOnTypeParameters.fir.kt

@@ -1,16 +0,0 @@
-// The type checker used to think that T is not null no matter what the upper bound
-
-fun <T, INDIRECT: T> nullableUpperBound(t: T, ind: INDIRECT) {
-    if (t == null) {} // was a warning
-    if (t != null) {}  // was a warning
-    if (ind == null) {}  // was a warning
-    if (ind != null) {}  // was a warning
-}
-
-fun <T: Any, INDIRECT: T> notNullUpperBound(t: T, ind: INDIRECT) {
-    if (t == null) {} // still a warning
-    if (t != null) {} // still a warning
-    if (ind == null) {} // still a warning
-    if (ind != null) {} // still a warning
-}
-

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/senslessComparisonWithNullOnTypeParameters.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // The type checker used to think that T is not null no matter what the upper bound
 
 fun <T, INDIRECT: T> nullableUpperBound(t: T, ind: INDIRECT) {