Preserve domtree and loop-simplify for runtime unrolling.

Preserve domtree and loop-simplify for runtime unrolling.

Mostly straightforward changes; we just didn't do the computation before.
One sort of interesting change in LoopUnroll.cpp: we weren't handling
dominance for children of the loop latch correctly, but
foldBlockIntoPredecessor hid the problem for complete unrolling.

Currently punting on loop peeling; made some minor changes to isolate
that problem to LoopUnrollPeel.cpp.

Adds a flag -unroll-verify-domtree; it verifies the domtree immediately
after we finish updating it. This is on by default for +Asserts builds.

Differential Revision: https://reviews.llvm.org/D28073

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292447 91177308-0d34-0410-b5e6-96231b3b80d8