[VLIWMachineScheduler] Disable default copy ctor and copy assignment operator for VLIWSchedBoundary

class VLIWSchedBoundary manages resources such as dynamically allocated memory, it's generally a good practice to either implement a custom copy constructor or disable the default one.

Reviewed By: JamesNagurne

Differential Revision: https://reviews.llvm.org/D151699