[PATCH][Review Requested][Compilation Time] Speed up SelectionDAG by hashing according to the node type

[Gurd, Preston]

Instead of deriving the selection DAG node class from the FoldingSetNode class, this patch derives it from the new SDNodeItem and SDNodeSet classes.

The purpose of the change is to be able to calculate the CSEMap hash table entries faster, by using a hash based on the node itself, rather than by using the hash function of FoldingSetNode.

Patch by Wan Xiaofei.

PLEASE REVIEW. Thanks!

--
Preston Gurd <preston.gurd at intel.com>
  Intel Waterloo
  SSG/DPD/ECDL/DMP

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