[VPlan] Recipe for splat operation

[Mel-Chen]

While trying to re-implement min/max with index, I discovered this potential bug.

middle.block:
  EMIT vp<%8> = compute-reduction-result ir<%max.09>, ir<%1>
  EMIT vp<%9> = icmp eq ir<%1>, vp<%8>

Currently, it seems there’s no check to ensure the vector preheader dominates the definition before hoisting the splat to the vector preheader. This could result in a use-before-definition issue.

For now, I’ve temporarily reverted SafeToHoist to an earlier setting, but I think the best approach would likely be to introduce a dedicated VPInstruction::Splat.