[DA] DivergenceAnalysis for unstructured, reducible CFGs

Summary:
This is patch 2 of the new DivergenceAnalysis (https://reviews.llvm.org/D50433).

This patch contains a generic divergence analysis implementation for
unstructured, reducible Control-Flow Graphs. It contains two new classes.
The `SyncDependenceAnalysis` class lazily computes sync dependences, which
relate divergent branches to points of joining divergent control. The
`DivergenceAnalysis` class contains the generic divergence analysis
implementation.

Reviewers: nhaehnle

Reviewed By: nhaehnle

Subscribers: sameerds, kristina, nhaehnle, xbolva00, tschuett, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D51491

llvm-svn: 344734