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

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

This issue was also pointed out when reviewing https://github.com/llvm/llvm-project/pull/119510, but it would probably be better to fix it separately as in this PR, especially as it comes with a test case.

Does this mean we have some gaps in RISCV testing?

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