[MC][Tablegen] Allow models to describe the retire control unit for llvm-mca.

This patch adds the ability to describe properties of the hardware retire
control unit.

Tablegen class RetireControlUnit has been added for this purpose (see
TargetSchedule.td).

A RetireControlUnit specifies the size of the reorder buffer, as well as the
maximum number of opcodes that can be retired every cycle.

A zero (or negative) value for the reorder buffer size means: "the size is
unknown". If the size is unknown, then llvm-mca defaults it to the value of
field SchedMachineModel::MicroOpBufferSize.  A zero or negative number of
opcodes retired per cycle means: "there is no restriction on the number of
instructions that can be retired every cycle".

Models can optionally specify an instance of RetireControlUnit. There can only
be up-to one RetireControlUnit definition per scheduling model.

Information related to the RCU (RetireControlUnit) is stored in (two new fields
of) MCExtraProcessorInfo.  llvm-mca loads that information when it initializes
the DispatchUnit / RetireControlUnit (see Dispatch.h/Dispatch.cpp).

This patch fixes PR36661.

Differential Revision: https://reviews.llvm.org/D45259

llvm-svn: 329304