[IslAst] Untangle IslAst lit-testcases from specifics of the legacy-PM

Summary:
This consists instances of two changes:

- Accept any order of checks for a specific loop form, that appear in different order in the new vs legacy-PM.
- Remove checks for specific regions.

Reviewers: grosser

Reviewed By: grosser

Subscribers: pollydev, llvm-commits

Tags: #polly

Differential Revision: https://reviews.llvm.org/D35837

llvm-svn: 308976