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

[Jeffrey Byrnes via llvm-commits]

jrbyrnes wrote:

Only user of this flag that I see is in `X86/sink-hoist.ll` and the port has no effect https://github.com/llvm/llvm-project/blob/main/llvm/test/CodeGen/X86/sink-hoist.ll

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