Revert "[ValutTracking] Use isGuaranteedNotToBePoison in impliesPoison"

This broke two lit tests:

  LLVM :: Transforms/LoopSimplify/merge-exits.ll
  LLVM :: Transforms/PhaseOrdering/X86/vector-reductions.ll

see comment on the code review.

> Differential Revision: https://reviews.llvm.org/D149934

This reverts commit 2ba4cfd5.