[PGO] Value profiling (index format) code cleanup and testing

 1. Added a set of public interfaces in InstrProfRecord
    class to access (read/write) value profile data.
 2. Changed IndexedProfile reader and writer code to 
    use the newly defined interfaces and hide implementation
    details.
 3. Added a couple of unittests for value profiling:
   - Test new interfaces to get and set value profile data
   - Test value profile data merging with various scenarios.

 No functional change is expected. The new interfaces will also
 make it possible to change on-disk format of value prof data
 to be more compact (to be submitted). 

llvm-svn: 251771