Cache the incremental iterators as you traverse the list, so that you don't...

Cache the incremental iterators as you traverse the list, so that you don't have to keep recomputing them

If memory turns out to be a problem, which I don't think it will in practice because all these ValueObjects, we'd be keeping alive anyway, I can always resort to caching the farthest-most iterator only

This gains us an order of magnitude in my benchmark, cutting the time to traverse a 1500-elements list from 22 seconds down to 2

llvm-svn: 254762