Can PForUtil be further auto-vectorized?

While working on LUCENE-9850, we discovered the loop in PForUtil::prefixSumOf is not getting auto-vectorized by the HotSpot compiler. We tried a few different tweaks to see if we could change this, but came up empty. There are some additional suggestions in the related PR that could still be experimented with, and it may be worth doing so to see if further improvements could be squeezed out.