[ViewCFG] Allow printing edge weights in debuggers

Summary:
Extending the Function::viewCFG prototypes to allow for printing block probability info in form of .dot files during debugging.

Also avoiding an AV when no BFI/BPI available.

Reviewers: wenlei, davidxl, knaumov

Reviewed By: wenlei, davidxl

Subscribers: MaskRay, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77978