[DDG] Data Dependence Graph - Add query function for memory dependencies between two nodes

Summary:
When working with the DDG it's useful to be able to query details of the
memory dependencies between two nodes connected by a memory edge. The DDG
does not hold a copy of the dependencies, but it contains a reference to a
DependenceInfo object through which dependence information can be queried.
This patch adds a query function to the DDG to obtain all the Dependence
objects that exist between instructions of two nodes.

Authored By: bmahjour

Reviewers: Meinersbur, Whitney, etiotto

Reviewed By: Whitney

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80529