[block-freq] Begin move of BlockFrequency::EntryFreq -> BlockFrequencyImpl::EntryFreq.

This is a property associated with a function, not with BlockFrequency data.
Additionally it loosens the artifical requirement that the entry frequency
arbitrarily be the same for every function.

There is a series of patches forthcoming updating various code that uses the old
way of getting a block frequency to the new location.

llvm-svn: 197284