[Support] Add general isl tools for DeLICM. NFC.

Add some generally useful isl tools into a their own new ISLTools.cpp.
These are the helpers were extracted from and will be use by the DeLICM
algorithm (https://reviews.llvm.org/D24716

).

Suggested-by: Tobias Grosser <tobias@grosser.es>
llvm-svn: 293340