[X86][SSE] SimplifyDemandedVectorEltsForTargetNode - reduce vector width of X86ISD::BLENDI

If we don't need the upper subvector elements of the BLENDI node then use a smaller vector size.

This causes a couple of minor regressions in insertelement-ones.ll which are more examples of PR26018; given how cheap allones generation is I don't consider that a showstopper, just an annoyance (and there's plenty of other poor codegen cases in that file).