FIR: Fix incorrectly serialized type

See the test added: there's a non-denotable T!! type inside flexible type
that wasn't handled before.

ConeKotlinType::contains handles flexible types content and some other cases
Also, it has better asymptotics

compiler/tests-gen/org/jetbrains/kotlin/codegen/LightAnalysisModeTestGenerated.java

@@ -27298,6 +27298,11 @@ public class LightAnalysisModeTestGenerated extends AbstractLightAnalysisModeTes
             runTest("compiler/testData/codegen/box/regressions/approximateIntersectionType.kt");
         }
 
+        @TestMetadata("approximationForDefinitelyNotNull.kt")
+        public void testApproximationForDefinitelyNotNull() throws Exception {
+            runTest("compiler/testData/codegen/box/regressions/approximationForDefinitelyNotNull.kt");
+        }
+
         @TestMetadata("arrayLengthNPE.kt")
         public void testArrayLengthNPE() throws Exception {
             runTest("compiler/testData/codegen/box/regressions/arrayLengthNPE.kt");