Revert "Revert r270478 "[LoopUnroll] Enable advanced unrolling analysis by default.""

This reverts commit r270512 and reapplies r270478. Originally it caused
PR27847, but it was fixed in r270517.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@270518 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed