StructurizeCFG: Use LoopInfo analysis for better loop detection

We were assuming that each back-edge in a region represented a unique
loop, which is not always the case.  We need to use LoopInfo to
correctly determine which back-edges are loops.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223199 91177308-0d34-0410-b5e6-96231b3b80d8