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

[Luke Lau via llvm-commits]

lukel97 wrote:

> Does this mean we have some gaps in RISCV testing?

Most likely! There's a separate assertion failure in inferScalarTypes that I'm running to after this PR. I'm bisecting it currently, I think there's some sort of non-deterministic clobbering of the cached types going on? 

In any case it might be a good idea to start giving any EVL-related PRs a quick build on llvm-test-suite/SPEC, or to keep an eye out on the RISC-V EVL buildbot: https://lab.llvm.org/buildbot/#/builders/132

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