Use a points-to graph for alias analysis (#16386)

Use a points-to graph for alias analysis (#16386)

Summary:
This PR changes the way we store aliasing information from a "set" approach to a "points-to" analysis. Set-based approaches lose information in ways that make it difficult to do "live" updates to the alias DB as one as mutating the graph.

The tradeoff is that simple queries get more expensive, since they require traversing the points-to graph to answer most questions. In practice, this is unlikely to be that costly since we don't have massive aliasing chains, but we could create an approximation/caching layer if this becomes a problem.

My rough plan is:
1. This PR, switching to a points-to graph
2. Make it "live": analyzing a node should record all the edges the node added, so that we can rollback when the node is destroyed.
3. Reduce wildcard scope: we can make the wildcard a special vertex that points to anything that we're not "sure" about; namely, things that have been put inside lists, or graph inputs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16386

Differential Revision: D13855117

Pulled By: suo

fbshipit-source-id: f009f58143173c275501624eb105d07ab60fe5e1