PERF: avoid copy in concatenate_array_managers if reindex already copies
@jbrockmendel https://github.com/pandas-dev/pandas/pull/42797 added an unconditional copy, while if you reindex the arrays, you already are sure you have a copy. 

This gives a 20% improvement on some of the merge benchmarks.
lgtm @jbrockmendel comment could be addressed here