[DAG] add splat vector support for 'or' in SimplifyDemandedBits

I've changed one of the tests to not fold away, but we didn't and still don't do the transform
that the comment claims we do (and I don't know why we'd want to do that).

Follow-up to:
https://reviews.llvm.org/rL300725
https://reviews.llvm.org/rL300763

llvm-svn: 300772