Some cleanup and bulletproofing for node replacement in LegalizeDAG. To...

Some cleanup and bulletproofing for node replacement in LegalizeDAG.  To maintain LegalizeDAG invariants, whenever we a node is replaced, we must attempt to delete it, and if it still
has uses after it is replaced (which can happen in rare cases due to CSE), we must revisit it.

llvm-svn: 144432