TableGen sometimes synthesizes missing sub-register indexes. Emit these
indexes as enumerators in the target namespace along with the
user-defined ones.
Also take this opportunity to stop creating new Record objects for
synthetic indexes.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161964 91177308-0d34-0410-b5e6-96231b3b80d8