Compute lists of sub-regs, super-regs, and overlapping regs.

Besides moving structural computations to CodeGenRegisters.cpp, this
also well-defines the order of these lists:

- Sub-register lists come from a pre-order traversal of the graph
  defined by the SubRegs lists in the .td files.

- Super-register lists are topologically ordered so no register comes
  before any of its sub-registers. When the sub-register graph is not a
  tree, independent super-registers appear in numerical order.

- Lists of overlapping registers are ordered according to register
  number.

This reverses the order of the super-regs lists, but nobody was
depending on that. The previous order of the overlaps lists was odd, and
it may have depended on the precise behavior of std::stable_sort.

The old computations are still there, but will be removed shortly.

llvm-svn: 132881