[llvm] [SandboxVec][Scheduler][DAG] Fix the update of DGNode on setOperand (PR #214110)

[via llvm-commits]

vporpo wrote:

I modified the patch and made it a lot simpler. Instead of moving the callback to the scheduler i just made the DAG aware of the direction.

https://github.com/llvm/llvm-project/pull/214110