Reapply [InstCombine] Support inverting lshr with non-negative operand

My initial patch contained a typo, resulting in the wrong value
being checked for non-negativeness.

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If the lshr operand is non-negative, we can treat it the same
way as an ashr. Ideally we would represent this as "lshr nneg",
but for now just perform the necessary ValueTracking query.

Proof: https://alive2.llvm.org/ce/z/Ahg4ri