[llvm] [LV] Set the underlying value of VPInstruction::Reverse (PR #173856)

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

> @alexey-bataev @fhahn I think #125640 can address this issue. However, it involves more changes, so it’s unclear whether it can land soon.
> 
> For now, I have an alternative approach that doesn’t require setting the underlying instruction. We could collect exit recipes when constructing VPCostContext, and add VPCostContext::skipCostComputation(VPRecipeBase *R, bool IsVector) to indicate that all exit recipes should be skipped. This way, VPRecipeBase::cost can ignore reverse recipes that serve as exit instructions.
> 
> What do you think?

@fhahn?

https://github.com/llvm/llvm-project/pull/173856