Track IR ordering of SelectionDAG nodes 1/4.

Use a field in the SelectionDAGNode object to track its IR ordering.
This adds fields and utility classes without changing existing
interfaces or functionality.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182701 91177308-0d34-0410-b5e6-96231b3b80d8