SI-5568 Fixes verify error from getClass on refinement of value type

().asInstanceOf[AnyRef with Unit].getClass and
5.asInstanceOf[AnyRef with Int].getClass would cause a verify
error. Going the other way, i.e. [Unit with AnyRef] or [Int with AnyRef]
worked fine. This commit fixes it that both directions work out to
BoxedUnit or java.lang.Integer.
SI-5568 Comment improvements for getClass on primitive intersection.

Based on code review here are a few comment cleanups and the removal of
some dead test code.
Merge pull request #1904 from JamesIry/SI-5568_2.10.x

SI-5568 Fixes verify error from getClass on refinement of value type