CLN: use _values_for_argsort for join_non_unique, join_monotonic
With the `.copy()` removed from `Categorical._values_for_argsort`, `ea_backed_index._data._values_for_argsort()` matches `ea_backed_index._ndarray_values` in all extant cases.

cc @jorisvandenbossche @TomAugspurger need to confirm 

a) this is an intended-adjacent use of _values_for_argsort, and not just a coincidence that it matches extant behavior
b) the `.copy()` this removes from `Categorical._values_for_argsort` is not important for some un-tested reason

xref #32452, #32426
I believe the only requirement on values_for_argsort is that it's a monotonic transformation (it preserves the ordering).

I'm not sure why that copy was there. > I believe the only requirement on values_for_argsort is that it's a monotonic transformation (it preserves the ordering).

It isn't clear to me what distinguishes this from values_for_factorize, as the docstring there also says `An array suitable for factorization. This should maintain order and be a supported dtype`.  (the "supported dtype" bit isnt in the values_for_argsort docstring, but seems implied since it returns an ndarray)

This also came up in #30673 (attempt to implement value_counts in terms of EA methods).  See also #32412.

Do we know of any 3rd-party EAs where _ndarray_values, values_for_argsort() and _values_for_factorize()[0] are meaningfully distinct? are there any user facing things that this now allows? e.g. joins on EA? > are there any user facing things that this now allows? e.g. joins on EA?

behavior is unchanged thanks, pls followon with consoilidations when you can My *feeling* says that this should use `_values_for_factorize`, as joining is a factorize-based algo? > My feeling says that this should use _values_for_factorize, as joining is a factorize-based algo?

I think you're right, but ATM _ndarray_values matches values_for_argsort for all our Index-backing EAs, but values_for_factorize()[0] is slightly different for DTA/TDA (_data vs asi8).  If we determine that we can change DTA/TDA _values_for_factorize (possibly as part of the discussion in #32586) then ill switch over these usages.