Adapt to DomTree changes in r199104

This fixes the compilation of our GPGPU code generation experiment in Polly.

Contributed-by: Yabin Hu <yabin.hwu@gmail.com>

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@208287 91177308-0d34-0410-b5e6-96231b3b80d8