[Polly] Partially refactoring of IslAstInfo and IslNodeBuilder to use isl++. NFC.
Polly use algorithms from the Integer Set Library (isl), which is a library written in C and which is incompatible with the rest of the LLVM as it is written in C++.
- Refactoring the following methods of class IslAstInfo
- isParallel() isExecutedInParallel() isReductionParallel() getSchedule() getMinimalDependenceDistance() getBrokenReductions()
- Refactoring the following methods of class IslNodeBuilder
- getReferencesInSubtree() getScheduleForAstNode()
- Refactoring function getBrokenReductionsStr()
- Fixed the mismatching function declaration for getScheduleForAstNode()
Reviewed By: Meinersbur
Differential Revision: https://reviews.llvm.org/D99971