[block-freq] Add a print method on BlockFrequencyImpl for printing block frequencies and a convenience method for the common case of getting/printing a basic block.

BlockFrequencies can only be printed relative to their entry frequency. Thus
since the entry frequency is no longer necessarily a static constant on the
BlockFrequency class and is instead a potentially dynamic value taken from
BlockFrequencyImpl, we must necessarily print it via a method on
BlockFrequencyImpl.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197285 91177308-0d34-0410-b5e6-96231b3b80d8