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/ir/ir.tree/src/org/jetbrains/kotlin/ir/types/IrTypeSystemContext.kt

@@ -77,6 +77,9 @@ interface IrTypeSystemContext : TypeSystemContext, TypeSystemCommonSuperTypesCon
 
     override fun SimpleTypeMarker.isMarkedNullable(): Boolean = (this as IrSimpleType).hasQuestionMark
 
+    override fun KotlinTypeMarker.isMarkedNullable(): Boolean =
+        this is IrSimpleType && !isWithFlexibleNullability() && hasQuestionMark
+
     override fun SimpleTypeMarker.withNullability(nullable: Boolean): SimpleTypeMarker {
         val simpleType = this as IrSimpleType
         return if (simpleType.hasQuestionMark == nullable) simpleType