When the ProcResGroup has BufferSize=0, treat it as if the
contained resources are also using a group resource instance,1) if there is a subunit in the list of write resources
which can be requested separately. Used to model parallel for the scheduling class, do not attempt to schedule
uses from a pool of resources.
Note that this is a rework of D94604 without adding a new scheduling model annotation type 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 list of write resources for the scheduling
class is not supported.
There is a use of ProcResGroup with BufferSize=0 in revision: https://reviews.llvm.org/D98977Used to model parallel uses from a pool of resources.