[InstCombine] don't automatically drop poison-generating flags in SimplifyVectorDemandedElts

I noticed this while reviewing the test diffs in D115460
(and so the diffs in that patch will be reduced if this one is applied first).

This is effectively a revert of 3436dc29 ( https://reviews.llvm.org/rG3436dc29239d ) -
since that commit, we've made several enhancements, so the reasoning there is no longer
valid. Specifically, we added a poison value to IR, and we clarified the behavior of
undef/poison elements in a shuffle mask:
https://llvm.org/docs/LangRef.html#shufflevector-instruction

Alive2 seems to agree that the propagation of flags in the test diffs shown here are valid:
https://alive2.llvm.org/ce/z/UuY-jr
https://alive2.llvm.org/ce/z/GXoMD9
https://alive2.llvm.org/ce/z/nVCyVH

Differential Revision: https://reviews.llvm.org/D115526