[VPlan] Lower CanIVIncrementForPart in convertToConcreteRecipes. (#190844)
Move the lowering of CanonicalIVIncrementForPart from generate() to
convertToConcreteRecipes, converting it to an Add VPInstruction at the
VPlan level. This enables VPlan-level simplifications (e.g., folding add
0, x) and prepares for adding a 3-operand form.
PR: https://github.com/llvm/llvm-project/pull/190844