misched: Generic tablegen classes for the new machine model.

misched: Generic tablegen classes for the new machine model.

This is mostly documentation for the new machine model. It is designed
to be flexible, easy to incrementally refine for a subtarget, and
provide all the information that MachineScheduler will need.

If all goes well, I will follow up with an example of the new model in
use for ARM.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@163877 91177308-0d34-0410-b5e6-96231b3b80d8