blockfreq: Further shift logic to LoopData

Move a lot of the loop-related logic that was sprinkled around the code
into `LoopData`.

<rdar://problem/14292693>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207258 91177308-0d34-0410-b5e6-96231b3b80d8