[llvm] [AArch64][SVE2] Use rshrnb for masked stores (PR #70026)

[David Sherwood via llvm-commits]

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

Thanks for this @MDevereau - looks like another nice improvement! I just have one comment about potentially refactoring the code ...

https://github.com/llvm/llvm-project/pull/70026