[LoopUnroll] Use Lazy strategy for DTU used for MergeBlockIntoPredecessor.

We do not access the DT in the loop, so we do not have to apply updates
eagerly. We can apply them lazyly and flush them after we are done
merging blocks.

As follow-up work, we might be able to use the DTU above as well,
instead of manually updating the DT.

This brings the example from PR43134 from ~100s to ~4s for a relase +
assertions build on my machine.

Reviewers: efriedma, kuhar, asbirlea, brzycki

Reviewed By: kuhar, brzycki

Differential Revision: https://reviews.llvm.org/D66911

llvm-svn: 370292