[SDAG] Refactor the code which deletes nodes in the DAG combiner to do
so using a single helper which adds operands back onto the worklist.

Several places didn't rigorously do this but a couple already did.
Factoring them together and doing it rigorously is important to delete
things recursively early on in the combiner and get a chance to see
accurate hasOneUse values. While no existing test cases change, an
upcoming patch to add DAG combining logic for PSHUFB requires this to
work correctly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214623 91177308-0d34-0410-b5e6-96231b3b80d8