[DependenceInfo] Simplify use of StmtSchedule's domain [NFC]

Once a StmtSchedule is created, only its domain is used anywhere within
DependenceInfo::calculateDependences. So, we choose to return the
wrapped domain of the union_map rather than the entire union_map.

However, we still build the union_map first within collectInfo(). It is
cleaner to first build the entire union_map and then pull the domain out in
one shot, rather than repeatedly extracting the domain in bits and pieces
from accdom.

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

Differential Revision: https://reviews.llvm.org/D30208

llvm-svn: 295984