[llvm] [VPlan] Passing non-null instruction when creating VPReductionRecipe in unit test. NFC (PR #120053)

[Elvis Wang via llvm-commits]

ElvisWang123 wrote:

I think we need to pass non-null underlying instruction when creating VPReductionRecipe in current implementation. 
Otherwise the `getUnderlyingInstr()`  which is used in `print()` and `clone()` will crash. 

We can add another constructor for VPReductionRecipe without underlying instruction in the future.


https://github.com/llvm/llvm-project/pull/120053