MachineModel: Add a ProcResGroup class.

This allows abitrary groups of processor resources. Using something in
a subset automatically counts againts the superset. Currently, this
only works if the superset is also a ProcResGroup as opposed to a
SuperUnit.

This allows SandyBridge to be expressed naturally, which will be
checked in shortly.

def SBPort01 : ProcResGroup<[SBPort0, SBPort1]>;
def SBPort15 : ProcResGroup<[SBPort1, SBPort5]>;
def SBPort23  : ProcResGroup<[SBPort2, SBPort3]>;
def SBPort015 : ProcResGroup<[SBPort0, SBPort1, SBPort5]>;