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

[David Green via llvm-commits]

https://github.com/davemgreen approved this pull request.

The cost and AArch64 parts look OK to me. (But I'm not married to the name if someone has a better suggestion).

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