[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 follows on from https://github.com/llvm/llvm-project/pull/143526, where I discovered an issue with the vectoriser cost model regarding the cost of last lane extraction.

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