JVM IR: fix isMarkedNullable for nullability-flexible types

Since nullability-flexible types in IR are represented by an annotation
on an IrSimpleType, not a special instance, the common implementation of
`KotlinTypeMarker.isMarkedNullable` in `TypeSystemContext` doesn't work.

This method is used for example to generate `typeOf` in JVM IR, in
`TypeSystemCommonBackendContext.generateTypeOf`, and this issue led to a
difference in behavior of `typeOf` for nullability-flexible types.

 #KT-44726 Fixed

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

@@ -28146,6 +28146,11 @@ public class LightAnalysisModeTestGenerated extends AbstractLightAnalysisModeTes
                 runTest("compiler/testData/codegen/box/reflection/typeOf/classes.kt");
             }
 
+            @TestMetadata("flexibleType.kt")
+            public void testFlexibleType() throws Exception {
+                runTest("compiler/testData/codegen/box/reflection/typeOf/flexibleType.kt");
+            }
+
             @TestMetadata("inlineClasses.kt")
             public void testInlineClasses() throws Exception {
                 runTest("compiler/testData/codegen/box/reflection/typeOf/inlineClasses.kt");