Update the recommended isl version

Recent changes in isl:

- Allow analysis of loops during code generation

This simplifies the detection of parallel loops.

- Simplify the way costumized ast printers are defined

This enables us to highlight parallel / vector loops in our debug output.

- Compile time improvements for codegen contexts that include parameters

- Various bug fixes

This update also gets us in sync for the isl 0.11 release.

Merged from: https://llvm.org/svn/llvm-project/polly/trunk@169100

llvm-svn: 170746