[llvm] [Analysis] Fix null ptr dereference when using WriteGraph without branch probability info (PR #104102)

[Tyler Nowicki via llvm-commits]

TylerNowicki wrote:

The call to 'CFGInfo->getBPI()->getEdgeProbability(Node, SuccBB);' fails when BPI is not provided. This caused WriteGraph to fail, even though it should support null BPI. This patch makes getEdgeAttribute give up if BPI is not present.

https://github.com/llvm/llvm-project/pull/104102