Add a new TargetInstrInfo MachineInstr opcode, COPY_TO_SUBCLASS.

This will be used to replace things like X86's MOV32to32_.

Enhance ScheduleDAGSDNodesEmit to be more flexible and robust
in the presense of subregister superclasses and subclasses. It
can now cope with the definition of a virtual register being in
a subclass of a use.

Re-introduce the code for recording register superreg classes and
subreg classes. This is needed because when subreg extracts and
inserts get coalesced away, the virtual registers are left in
the correct subclass.

llvm-svn: 68961