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.

llvm-svn: 29550