Fix comparing AnyVal | Null to Null and selecting in UnsafeNulls

[noti0na1]

When typing a Select under unsafeNulls, we should only strip Null from T | Null if T <:< ObjectType, so that unsafeNulls will have same behaviours as regular scala without explicit nulls.

This PR fixs comparing AnyVal | Null with Null using == and != in unsafeNulls:

val i: Int | Null = 0
if i != null then // ok
  println(i)

It also disallows selecting members of AnyVal on AnyVal | Null:

case class MyVal(i: Int) extends AnyVal:
  def printVal: Unit =
    println(i)

val v: MyVal | Null = MyVal(1)

import scala.language.unsafeNulls

v.printVal // error: value printVal is not a member of MyVal | Null