[llvm] [VPlan] Explicitly express the shift immediate for splice operation. NFC (PR #123923)

[Florian Hahn via llvm-commits]

fhahn wrote:

> > Is the only purpose to avoid passing `Plan` to `createEVLRecipe`?
> 
> Yes.
> 
> > Could we instead make it easier to create live-in constants, maybe something like `VPlan::getOrAddInt32(int32_t Value)`? A `VPlan` object can be retrieved from the recipe itself.
> 
> Hmm, maybe I am a little overthinking. Do you feel [this](https://github.com/llvm/llvm-project/pull/124093/commits/e7b6d6eb069d32127b9441b12374c157ba75ad83) is good enough, right? 

e7b6d6eb069d32127b9441b12374c157ba75ad83 looks good to me

https://github.com/llvm/llvm-project/pull/123923