[CodeGen] Use ProcResGroup information in SchedBoundary

When the ProcResGroup has BufferSize=0,

1. if there is a subunit in the list of write resources for the
   scheduling class, do not attempt to schedule the ProcResGroup.
2. if there is not a subunit in the list of write resources for the
   scheduling class, choose a subunit to use instead of the ProcResGroup.
3. having both the ProcResGroup and any of its subunits in the resources
   implied by a InstRW is not supported.

Used to model parallel uses from a pool of resources.

Differential Revision: https://reviews.llvm.org/D98976