Revert "PERF: tighten _should_compare for MultiIndex"
Reverts pandas-dev/pandas#42231

This introduced a bug discussed in #42231.  The long-term fix is in MultiIndex.get_indexer, but that's going to take me a little while to get done.
@simonjayhawkins @jreback should we try to get this in for 1.3.1? #42231 (source of regression) is not on 1.3.x? woops, assumed bc we got a bug report it must have been released, but not so thanks @jbrockmendel 