No need to run -mem2reg twice

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@214632 91177308-0d34-0410-b5e6-96231b3b80d8
Mark a GPGPU test case as XFAIL

This area of code is currently not very much tested. It will hopefully be
superseeded by Yabin's GSoC project.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@214633 91177308-0d34-0410-b5e6-96231b3b80d8