[X86][TLI] SimplifyDemandedVectorEltsForTargetNode(): don't break apart...

[X86][TLI] SimplifyDemandedVectorEltsForTargetNode(): don't break apart broadcasts from which not just the 0'th elt is demanded

Apparently this has no test coverage before D108382,
but D108382 itself shows a few regressions that this fixes.

It doesn't seem worthwhile breaking apart broadcasts,
assuming we want the broadcasted value to be preset in several elements,
not just the 0'th one.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D108411