Update Lint/UnmodifiedReduceAccumulator to accept blocks which return in the form accumulator[element]

[dvandersluis]

Reduce blocks that just return something in the form accumulator[element] are now accepted by Lint/UnmodifiedReduceAccumulator (the issue asked for all methods on the accumulator to be allowed but this was the only exception previously due to the check for accumulator index). Other accumulator indexes are still offenses.

Fixes #9059.

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