[PGO] Make indexed value profile data more compact

[PGO] Make indexed value profile data more compact

- Make indexed value profile data more compact by peeling out
  the per-site value count field into its own smaller sized array.
- Introduced formal data structure definitions to specify value
  profile data layout in indexed format. Previously the layout
  of the data is only assumed in the client code (scattered in
  three different places : size computation, EmitData, and ReadData
- The new data structure  serves as a central place for layout documentation.
- Add interfaces to force BE output for value profile data (testing purpose)
- Add byte swap unit tests

Differential Revision: http://reviews.llvm.org/D14401

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252563 91177308-0d34-0410-b5e6-96231b3b80d8