* Enable SCCP debugging to be turned on with a simple change of #define

* Change worklist to a set so that duplicates cannot happen
* Add support for the invoke instruction
* Avoid marking store and free instructions as overdefined, since they
  cannot produce a value anyway.
* Inline the OperandChangedState implementation
* Add isEdgeFeasible in preparation to fix a bug.  Right now it behaves
  exactly as before.
* Remove obsolete comment about constant pools

llvm-svn: 2445