[LoopDeletion] (cleanup, NFC) Use the dedicated helper to get a single

unique exit block if available rather than rolling it ourselves.

This is a little disappointing because that helper doesn't do anything
clever to short-circuit the (surprisingly expensive) computation of all
exit blocks. What's worse is that the way we compute this is hopelessly,
hilariously inefficient. We're literally computing the same information
two different ways and multiple times each way:
- hasDedicatedExits computes the exit block set and then looks at the
  predecessors of each
- getExitingBlocks computes the set of loop blocks which have exiting
  successors
- getUniqueExitBlock(s) computes the set of non-loop blocks reached from
  loop blocks (sound familiar?)

Anyways, at some point we should clean all of this up in the LoopInfo
API, but for now just simplifying the user I'm about to touch.

llvm-svn: 292282