Expose CFG Update struct. Define GraphTraits to get children given a snapshot CFG.

Summary:
Certain passes or analysis need to view a CFG snapshot rather than the actual CFG. This patch provides GraphTraits to offer such a view.

The patch defines GraphTraits for BasicBlock* and Inverse<BasicBlock*> to provide CFG successors and predecessors based on a list of CFG updates.

An Update is defined as a triple {InsertOrDeleteKind, BlockStartOfEdge, BlockEndOfEdge}.
A GraphDiff is defined as a list of Updates that has been preprocessed to treat the CFG as a graph rather than a multi-graph. As such, there can only exist a single Update given two nodes. All duplicates will be filtered and Insert/Delete edges that cancel out will be ignored.
The methods GraphDiff exposes are:
- Determine if an existing child needs to be ignored, i.e. an Update exists in the correct direction to assume the removal of that edge.
- Return a list of new children to be considered, i.e. an Update exists in the correct direction for each child in the list to assume the insertion of that edge.

Reviewers: timshen, kuhar, chandlerc

Subscribers: sanjoy, jlebar, llvm-commits

Differential Revision: https://reviews.llvm.org/D50479

llvm-svn: 339689