Use a proper worklist for use-def traversal without holding onto an
iterator. Now that we process all IV uses, we need complete logic for
resusing existing derived IV defs. See HoistStep.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132103 91177308-0d34-0410-b5e6-96231b3b80d8