[VPlan] Explicitly handle scalar pointer inductions. (#83068)

Add a new PtrAdd opcode to VPInstruction that corresponds to
IRBuilder::CreatePtrAdd, which creates a GEP with source element type
i8.

This is then used to model scalarizing VPWidenPointerInductionRecipe by
introducing scalar-steps to model the index increment followed by a
PtrAdd.

Note that PtrAdd needs to be able to generate code for only the first
lane or for all lanes. This may warrant introducing a separate recipe
for scalarizing that can be created without relying on the underlying
IR.

Depends on https://github.com/llvm/llvm-project/pull/80271

PR: https://github.com/llvm/llvm-project/pull/83068