[DAG] SimplifyDemandedBits/SimplifyDemandedVectorElts - improve handling of multiple uses of MemSDNode chain

[RKSimon]

When calling both SimplifyDemandedBits/SimplifyDemandedVectorElts, we check if the entire node has multiple uses and then set the demandedbits/elts to ALL if it does.

But for memory loads (inc. MemIntrinsicSDNode) this behaves poorly with cases if all but one of the uses are for the memory chain - we should still be able to create a new MemSDNode (with adjusted memory width etc.) if we only use the loaded result once, as long as users of the chain are updated afterward.