Implement the first half of redundancy factoring: efficiently

Implement the first half of redundancy factoring: efficiently
splitting all the patterns under scope nodes into equality sets
based on their first node.  The second step is to rewrite the
graph info a form that exposes the sharing.  Before I do this,
I want to redesign the Scope node.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@97130 91177308-0d34-0410-b5e6-96231b3b80d8