[CodeGen][NFC] Declare copy constructor & copy assignment as deleted for ScheduleDAG

ScheduleDAG has derived classes ScheduleDAGVLIW and ScheduleDAGRRList,
which own resources that are freed in their destructors. Static analyzer
warns b/c they do not have user-written copy constructors.

According to the design of ScheduleDAG, it seems that it should always
be passed by reference. So I declare them as deleted in this patch.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D151538