isl: Detect openmp parallelism

Based on code written by Riyadh Baghdadi.

Merged from: https://llvm.org/svn/llvm-project/polly/trunk@170102

git-svn-id: https://llvm.org/svn/llvm-project/polly/branches/release_32@170753 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed