LoopVectorizer: Enable unrolling of conditional stores and the load/store
unrolling heuristic per default

Benchmarking on x86_64 (thanks Chandler!) and ARM has shown those options speed
up some benchmarks while not causing any interesting regressions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200621 91177308-0d34-0410-b5e6-96231b3b80d8