[llvm] [AArch64] Refactor creation of a shuffle mask for TBL (NFC) (PR #92529)

[Florian Hahn via llvm-commits]

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

LGTM, would be good to have a test case & remove the NFC tag if assert is replaced by early exit

https://github.com/llvm/llvm-project/pull/92529