Remove support for 'CompositeIndices' and sub-register cycles.

Now that the weird X86 sub_ss and sub_sd sub-register indexes are gone,
there is no longer a need for the CompositeIndices construct in .td
files. Sub-register index composition can be specified on the
SubRegIndex itself using the ComposedOf field.

Also enforce unique names for sub-registers in TableGen. The same
sub-register cannot be available with multiple sub-register indexes.

llvm-svn: 160842