[llvm] [AArch64] Use INDEX for constant Neon step vectors (PR #113424)

[Paul Walker via llvm-commits]

https://github.com/paulwalker-arm commented:

Please add a more specific/dedicated test because the existing active_lane_mask tests should all ideally result in `while` instructions when SVE is available, so we'll be missing test coverage when that happens.

https://github.com/llvm/llvm-project/pull/113424