[X86] Return SDValue(N, 0) instead of an SDValue() after a successful combine.

Returning SDValue() means nothing changed, SDValue(N,0) means there was a change but the worklist management was taken care of.

I don't know if this has a real effect other than making sure the combine counter in the DAG combiner gets updated, but it is the correct thing to do.

llvm-svn: 321463