Fix the LoopUnroller to handle nontrivial loops and partial unrolling.

Fix the LoopUnroller to handle nontrivial loops and partial unrolling.

These are not individual bug fixes. I had to rewrite a good chunk of
the unroller to make it sane. I think it was getting lucky on trivial
completely unrolled loops with no early exits. I included some fairly
simple unit tests for partial unrolling. I didn't do much stress
testing, so it may not be perfect, but should be usable now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@137190 91177308-0d34-0410-b5e6-96231b3b80d8