[AMDGPU] SIMachineScheduler: Add support for several MachineScheduler features

The SI machine scheduler inherits from ScheduleDAGMI.
This patch adds support for a few features that are implemented
in ScheduleDAGMI (or its base classes) that were missing so far
because their support is implemented in overridden functions.

* Support cl::opt -view-misched-dags
  This option allows to open a graphical window of the scheduling DAG.

* Support cl::opt -misched-print-dags
  This option allows to print the scheduling DAG in text form.

* After constructing the scheduling DAG, call postprocessDAG()
  to apply any registered DAG mutations.
  Note that currently there are no mutations defined in AMDGPUTargetMachine.cpp
  in case SIScheduler is used.
  Still add this to avoid surprises in the future in case mutations are added.

Differential Revision: https://reviews.llvm.org/D128808