[llvm] [ScheduleDAG] Allow disabling the SchedModel / Itineraries during Scheduling (PR #138057)

[Jeffrey Byrnes via llvm-commits]

jrbyrnes wrote:

> I don't know why you would want to only disable this during a specific pass

Certain passes may need this info for correctness. I've ported the existing flags into the Scheduler s.t. using them only works on scheduling pass

https://github.com/llvm/llvm-project/pull/138057