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Preserve domtree and loop-simplify for runtime unrolling.

Description

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