Added BlockFrequencyInfo::view for displaying the block frequency propagation graph via graphviz.

This is useful for debugging issues in the BlockFrequency implementation since
one can easily visualize where probability mass and other errors occur in the
propagation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194654 91177308-0d34-0410-b5e6-96231b3b80d8