[HotColdSplitting] Get DT and PDT from the pass manager.

Instead of manually computing DT and PDT, we can get the from the pass
manager, which ideally has them already cached. With the new pass
manager, we could even preserve DT/PDT on a per function basis in a
module pass.

I think this also addresses the TODO about re-using the computed DTs for
BFI. IIUC, GetBFI will fetch the DT from the pass manager and when we
will fetch the cached version later.

Reviewers: vsk, hiraditya, tejohnson, thegameg, sebpop

Reviewed By: vsk

Differential Revision: https://reviews.llvm.org/D57092

llvm-svn: 352036