[VPlan] Add new VPScalarCastRecipe, use for IV & step trunc. (#78113)

Add a new recipe to model scalar cast instructions, without relying on
an underlying instruction.

This allows creating scalar casts, without relying on an underlying
instruction (like the current VPReplicateRecipe). The new recipe is 
used to explicitly model both truncating the induction step and the
VPDerivedIVRecipe, thus simplifying both the recipe and code
needed to introduce it.

Truncating VPWidenIntOrFpInductionRecipes should also be modeled using
the new recipe, as follow-up.

PR: https://github.com/llvm/llvm-project/pull/78113