[CFGPrinter] Display branch weight on the edges

Summary:
This is pretty useful especially in connection with
BFI's -view-block-freq-propagation-dags.  It helped me to track down the
bug that is being fixed in D24118.

While -view-block-freq-propagation-dags displays the high-level
information with static heuristics included (and block frequencies), the
new thing only shows the raw weight as presented by PGO without any of
the static estimates.  This helps to distinguished what has been
measured vs. estimated.

For the sample loop in D24118, -view-block-freq-propagation-dags=integer
looks like this:

https://reviews.llvm.org/F2381352

While with -view-cfg-only you can see the underlying branch weights:

https://reviews.llvm.org/F2392296

Reviewers: dexonsmith, bogner, davidxl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D24144

llvm-svn: 280442