Replace the SubRegSet tablegen class with a less error-prone mechanism.

A Register with subregisters must also provide SubRegIndices for adressing the
subregisters. TableGen automatically inherits indices for sub-subregisters to
minimize typing.

CompositeIndices may be specified for the weirder cases such as the XMM sub_sd
index that returns the same register, and ARM NEON Q registers where both D
subregs have ssub_0 and ssub_1 sub-subregs.

It is now required that all subregisters are named by an index, and a future
patch will also require inherited subregisters to be named. This is necessary to
allow composite subregister indices to be reduced to a single index.

llvm-svn: 104704