Add default index functor (an identity functor). You could use a

Add default index functor (an identity functor). You could use a
vector directly to get the same functionality but using a DenseMap
makes the code more readable IMO.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@16052 91177308-0d34-0410-b5e6-96231b3b80d8