MemAccInt: Do not strip pointer casts

This mirrors:

commit  https://llvm.org/svn/llvm-project/llvm/trunk@263462
Author: Michael Kuperstein <michael.kuperstein@gmail.com>
Date:   Mon Mar 14 18:34:29 2016 +0000

    [AliasSetTracker] Do not strip pointer casts when processing MemSetInst

and fixes the failure the above commit triggered in Polly.

llvm-svn: 263538