[llvm] [VPlan] Handle VPWidenCastRecipe without underlying value in EVL transform (PR #120194)

[Luke Lau via llvm-commits]

lukel97 wrote:

> Noting this down, I was able to bisect the inferScalarTypes assertion (not the assertion in this PR!) back to [b759020](https://github.com/llvm/llvm-project/commit/b759020cc85d023583c948aa93905e38897b2c0a)

I've posted a fix for this in #120252

https://github.com/llvm/llvm-project/pull/120194