[VPlan] Handle VPWidenCastRecipe without underlying value in EVL transform (#120194)
This fixes a crash that shows up when building SPEC CPU 2017 with EVL
tail folding on RISC-V.
A VPWidenCastRecipe doesn't always have an underlying value, and in the
case of this crash this happens whenever a widened cast is created via
truncateToMinimalBitwidths.
Fix this by just using the opcode stored in the recipe itself.
I think a similar issue exists with VPWidenIntrinsicRecipe and how it's
widened, but I haven't run into any crashes with it just yet.