Patches for parametrized (variable) register classes are up for review

This is a continuation of the discussion started here:

The motivation was discussed in that thread, and I'll use Hexagon HVX here as a working example. HVX has two modes, where vector registers are 64- and 128-byte in size. The instruction set is identical in both cases, that includes the instruction and register encodings. As a matter of fact, it's possible to have a single binary that works in both modes.
The problem was that due to different register sizes, value types, etc., we had to create separate register classes for vector registers in the 64-byte mode and for the 128-byte mode. This forced us to implement two separate instruction sets that do the exact same thing.

I have posted a series of patches that allow parametrized register classes, where the register sizes, spill sizes, and value types can be predicated on a set of subtarget features. The parametrized value types can also be used in selection patterns.

With these patches we were able to completely remove the extra set of vector instructions from the Hexagon backend, and use a single set for both modes.

Preliminary reorganization of TargetRegisterClass members:

TableGen support:

Hexagon changes (showing how the above code can be used):

I have added some selection of people as reviewers, but please feel free to join in and comment.