Revamp the "CSEMap" datastructure used in the SelectionDAG class. This

Revamp the "CSEMap" datastructure used in the SelectionDAG class.  This
eliminates a bunch of std::map's in the SelectionDAG, replacing them with a
home-grown hashtable.

This is still a work in progress: not all the maps have been moved over and the
hashtable never resizes.  That said, this still speeds up llc 20% on kimwitu++
with -fast -regalloc=local using a release build.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@29550 91177308-0d34-0410-b5e6-96231b3b80d8