blockfreq: Embed Loop hierarchy in LoopData

blockfreq: Embed Loop hierarchy in LoopData

Continue refactoring to make `LoopData` first-class.  Here I'm making
the `LoopData` hierarchy explicit, instead of bouncing back and forth
with `WorkingData`.  This simplifies the logic and better matches the
`LoopInfo` design.  (Eventually, `LoopInfo` should be restructured so
that it supports this pass, and `LoopData` can be removed.)

<rdar://problem/14292693>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207180 91177308-0d34-0410-b5e6-96231b3b80d8