[X86] Remove some unnecessary explicit calls to DCI.AddToWorkList.

These calls were making sure some newly created nodes were added to worklist, but the DAGCombiner has internal support for ensuring it has visited all nodes. Any time it visits a node it ensures the operands have been queued to be visited as well. This means if we only need to return the last new node. The DAGCombiner will take care of adding its inputs thus walking backwards through all the new nodes.

llvm-svn: 337996