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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@163877 91177308-0d34-0410-b5e6-96231b3b80d8

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