[FIR] Properly handle flexible types during creation of DNNT

^KT-50788 Fixed

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosisCompilerFirTestdataTestGenerated.java

@@ -3533,6 +3533,12 @@ public class DiagnosisCompilerFirTestdataTestGenerated extends AbstractDiagnosis
                 runTest("compiler/fir/analysis-tests/testData/resolve/smartcasts/kt39000.kt");
             }
 
+            @Test
+            @TestMetadata("kt50788.kt")
+            public void testKt50788() throws Exception {
+                runTest("compiler/fir/analysis-tests/testData/resolve/smartcasts/kt50788.kt");
+            }
+
             @Test
             @TestMetadata("multipleCasts.kt")
             public void testMultipleCasts() throws Exception {