[DependenceInfo] Simplify creation and subsequent use of AccessSchedule [NFC]

We only ever use the wrapped domain of AccessSchedule, so stop
creating an entire union_map and then pulling the domain out.

Reviewers: grosser
Tags: #polly

Contributed-by: Siddharth Bhat <siddu.druid@gmail.com>

Differential Revision: https://reviews.llvm.org/D30179

llvm-svn: 295726