[llvm] [ARM][MVE] Add shuffle costs for LDn and STn instructions. (PR #145304)

[John Brawn via llvm-commits]

https://github.com/john-brawn-arm commented:

There's a lot of duplication between the LD and ST code (I've added comments to the LD code but they also apply to the ST code). I think it makes sense to move at least of the logic here into a separate function.

https://github.com/llvm/llvm-project/pull/145304