[CFGDiff] Refactor Succ/Pred maps.

Summary:
Refactor Succ/Pred maps to have a single map lookup when constructing
children. The preivous desing made sense when used by GraphTraits.
This more closely matches the previous approach in DomTree.

Reviewers: dblaikie

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D84567