to DenseMap<SDNode*, SUnit*>, and adjust the way cloned SUnit nodes are
handled so that only the original node needs to be in the map.
This speeds up llc on 447.dealII.llvm.bc by about 2%.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52576 91177308-0d34-0410-b5e6-96231b3b80d8