[X86] Add AddToWorklist(N) after calls to...

[X86] Add AddToWorklist(N) after calls to SimplifyDemandedBits/SimplifyDemandedVectorElts that are called on an operand of N.

If a simplication occurs the operand will be added to the worklist.
But since the demanded mask was based on N, we need to make sure
we revisit N in case there are more simplifications to be done.
Returning SDValue(N, 0) as we do, only tells DAG combine that
something changed, but that won't make it add anything to the
worklist.

Found while playing around with using VEXTRACT_STORE in more cases.
But I guess this doesn't affect any of our existing tests.