blockfreq: Use pointers to loops instead of an index

Store pointers directly to loops inside the nodes.  This could have been
done without changing the type stored in `std::vector<>`.  However,
rather than computing the number of loops before constructing them
(which `LoopInfo` doesn't provide directly), I've switched to a
`vector<unique_ptr<LoopData>>`.

This adds some heap overhead, but the number of loops is typically
small.

llvm-svn: 206857