Refactor/cleanup oopDesc::*_klass_addr

Right now this one has the comment mentioning CMS:

Klass** oopDesc::klass_addr(HeapWord* mem) {
  // Only used internally and with CMS and will not work with
  // UseCompressedOops
  assert(!UseCompressedClassPointers, "only supported with uncompressed klass pointers");
  ByteSize offset = byte_offset_of(oopDesc, _metadata._klass);
  return (Klass**) (((char*)mem) + in_bytes(offset));
}

But that is not true, it is used on the generic paths.

Maybe we can clean up related code a little too.