add a SelectionDAG method to check if no common bits are set in two nodes; NFCI

This was suggested in:
http://reviews.llvm.org/D13956

and is a follow-on to:
http://reviews.llvm.org/rL252515
http://reviews.llvm.org/rL252519

This lets us remove logically equivalent/duplicated code from DAGCombiner and X86ISelDAGToDAG.

A corresponding function for IR instructions already exists in ValueTracking.

llvm-svn: 252539