PollyDependence: Simplify Read/Write/MayWrite before feeding them into ISL.

Contributed-by: Star Tan <tanmx_star@yeah.net>

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@187981 91177308-0d34-0410-b5e6-96231b3b80d8