Fix KotlinType.isNullabilityFlexible for types based on type parameters

In the compiler, this function was used in psi2ir (and it affected
nullability annotations in the resulting JVM class files), in "useless
elvis with null on the right side" diagnostic checker, and in light
classes.

 #KT-42650 Fixed

core/descriptors/src/org/jetbrains/kotlin/types/flexibleTypes.kt

@@ -26,13 +26,12 @@ import org.jetbrains.kotlin.types.checker.KotlinTypeRefiner
 import org.jetbrains.kotlin.types.refinement.TypeRefinement
 import org.jetbrains.kotlin.types.typeUtil.builtIns
 
-
 fun KotlinType.isFlexible(): Boolean = unwrap() is FlexibleType
 fun KotlinType.asFlexibleType(): FlexibleType = unwrap() as FlexibleType
 
 fun KotlinType.isNullabilityFlexible(): Boolean {
     val flexibility = unwrap() as? FlexibleType ?: return false
-    return TypeUtils.isNullableType(flexibility.lowerBound) != TypeUtils.isNullableType(flexibility.upperBound)
+    return flexibility.lowerBound.isMarkedNullable != flexibility.upperBound.isMarkedNullable
 }
 
 // This function is intended primarily for sets: since KotlinType.equals() represents _syntactical_ equality of types,