JVM_IR fix cast to not-null type

We can't rely on argument type nullability here, because it still can
hold an uninitialized value on JVM.

KT-50577 KT-35272 KT-27427

native/native.tests/tests-gen/org/jetbrains/kotlin/konan/blackboxtest/ExternalTestGenerated.java

@@ -3487,6 +3487,12 @@ public class ExternalTestGenerated extends AbstractExternalNativeBlackBoxTest {
                 runTest("compiler/testData/codegen/box/casts/kt48927_privateMethodOnDerivedCastToBase.kt");
             }
 
+            @Test
+            @TestMetadata("kt50577.kt")
+            public void testKt50577() throws Exception {
+                runTest("compiler/testData/codegen/box/casts/kt50577.kt");
+            }
+
             @Test
             @TestMetadata("lambdaToUnitCast.kt")
             public void testLambdaToUnitCast() throws Exception {