[FIR] Consider Nothing? as null constant in DFA, ^KT-44561 Fixed

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

@@ -2,7 +2,7 @@ fun test() {
   val out : Int? = null
   val x : Nothing? = null
   if (out != x)
-    out<!UNSAFE_CALL!>.<!>plus(1)
+    out.plus(1)
   if (out == x) return
-  out<!UNSAFE_CALL!>.<!>plus(1)
+  out.plus(1)
 }

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

@@ -14,12 +14,12 @@ fun foo(x : String?, y : String?) {
     }
     else {
         // y == null but x != y
-        x<!UNSAFE_CALL!>.<!>length
+        x.length
         y<!UNSAFE_CALL!>.<!>length
     }
     if (y == null && x != y) {
         // y == null but x != y
-        x<!UNSAFE_CALL!>.<!>length
+        x.length
         y<!UNSAFE_CALL!>.<!>length
     }
     else {

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

@@ -3,7 +3,7 @@ fun foo(x: String?, y: String?, z: String?, w: String?) {
         z.length
     else
         z<!UNSAFE_CALL!>.<!>length
-    if (x != null || y != null || (x != z && y != z))
+    if (x != null || y != null || (x != z && <!SENSELESS_COMPARISON!>y != z<!>))
         z<!UNSAFE_CALL!>.<!>length
     else
         z<!UNSAFE_CALL!>.<!>length

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

@@ -20,6 +20,6 @@ fun bar(i: TestWithEquals?) {
 
 fun gav(i: TestWithEquals?, j: TestWithEquals?) {
     if (j == null) {
-        if (i == j) foo(<!ARGUMENT_TYPE_MISMATCH!>i<!>)
+        if (i == j) foo(i)
     }
 }