Make Loop::getExitBlocks significantly faster for large loops. Instead of

Make Loop::getExitBlocks significantly faster for large loops.  Instead of
pounding on Loop::contains (which is O(n) in the size of the loop), use a
sorted vector, which is O(log(N)) for each query.  This speeds up Duraid's
horrible testcase from ~72s to ~31s in a debug build.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@29645 91177308-0d34-0410-b5e6-96231b3b80d8