Use fetch_update instead of compare_exchange_weak
#1119
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Its manual `compare_exchange_weak` loop was exactly what `fetch_update` does internally, so we might as well simplify our code.
Its manual `compare_exchange_weak` loop was exactly what `fetch_update` does internally, and we can also improve on the case where the current best index was already equal.
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These are functionally equivalent, but
fetch_updatemakes our code simpler.Also, in the case of
find_first/last, it's now optimized that we don't need a compare-exchange at all when the best position is already equal. That may happen infind_lastwhen a result was already found in the current split, or in either method when the split resolution was exhausted.