[DepInfo] Use isl++ in Dependences::isValidSchedule. NFC.

Also change StatementToIslMapTy to hold isl::map, because it is used as a
parameter.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@339484 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed