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

[Sean Silva]

This patch has numerous coding standard
<http://llvm.org/docs/CodingStandards.html> violations, especially
variable naming and 80-columns.

> Patch by Wan Xiaofei.

Could you please have the original author send the patch so that they
can answer questions about the design and intent of the patch and
update it in response to reviewer comments?

-- Sean Silva