[llvm] [LV][TTI] Calculate cost of extracting last index in a scalable vector (PR #144086)

[David Sherwood via llvm-commits]

david-arm wrote:

This PR contains both a NFC patch to change the type of the index, and the follow-on patch to add support for calculating the cost of extracting the last index. I'm happy to split these into separate PRs if reviewers think that's better.

https://github.com/llvm/llvm-project/pull/144086