[DAGCombiner] remove explicit calls to AddToWorkList; NFCI
[DAGCombiner] remove explicit calls to AddToWorkList; NFCI

As noted in the post-commit thread for rL347917:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20181203/608936.html
...we don't need to repeat these calls because the combiner does it automatically.



git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@348597 91177308-0d34-0410-b5e6-96231b3b80d8

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