Revert "TableGen: Switch from a std::map to a DenseMap in CodeGenSubRegIndex. NFC"

It turns out we iterate over this map a fair amount and the order
matters for clang to be deterministic. See:

  http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160919/391315.html

This reverts r279875.

llvm-svn: 282040