[VPlan] Replace UnrollPart for VPScalarIVSteps with start index op (NFC) (#170906)
Replace the unroll part operand for VPScalarIVStepsRecipe with the start
index. This simplifies https://github.com/llvm/llvm-project/pull/170053
and is also a first step to break down the recipe into its components.
PR: https://github.com/llvm/llvm-project/pull/170906