[SelectionDAG] Respect multiple uses in SimplifyDemandedBits to SimplifyDemandedVecto...

[SelectionDAG] Respect multiple uses in SimplifyDemandedBits to SimplifyDemandedVectorElts simplification

rL343913 was using SimplifyDemandedBits's original demanded mask instead of the adjusted 'NewMask' that accounts for multiple uses of the op (those variable names really need improving....).

Annoyingly many of the test changes (back to pre-rL343913 state) are actually safe - but only because their multiple uses are all by PMULDQ/PMULUDQ.

Thanks to Jan Vesely (@jvesely) for bisecting the bug.

llvm-svn: 343935