[SI] Fix a potential use-after-free

Summary:
There is a potential use-after-free bug in Scop::buildSchedule(Region *,
LoopStackTy &, LoopInfo &).  Before, we took a reference to LoopStack.back()
which is a use after free, since back is popped off further below. This didn't
crash before by pure chance, since LoopStack is actually a vector, and the
memory isn't freed upon pop. I turned this into an iterator-based algorithm.

Reviewers: grosser, bollu, Meinersbur

Reviewed By: Meinersbur

Subscribers: llvm-commits, pollydev

Differential Revision: https://reviews.llvm.org/D39979

llvm-svn: 318415