Specify SubRegIndex components on the index itself.

It is simpler to define a composite index directly:

  def ssub_2 : SubRegIndex<[dsub_1, ssub_0]>;
  def ssub_3 : SubRegIndex<[dsub_1, ssub_1]>;

Than specifying the composite indices on each register:

  CompositeIndices = [(ssub_2 dsub_1, ssub_0),
                      (ssub_3 dsub_1, ssub_1)] in ...

This also makes it clear that SubRegIndex composition is supposed to be
unique.

llvm-svn: 149556