[llvm] [AArch64][ARM] Optimize more `tbl`/`tbx` calls into `shufflevector` (PR #169748)

[via llvm-commits]

valadaptive wrote:

> Can you rebase to make it clearer what has now changed? Cheers

I've rebased it on top of the code-review changes I made to https://github.com/llvm/llvm-project/pull/169589. It'll still show as including both sets of changes; in order to avoid that, I think I'd need to use Graphite, which requires commit access. To work around that, you can [review the new commit individually](https://github.com/llvm/llvm-project/pull/169748/commits/f9b878645cc4df65d4051cdc1a69f980ef05c2b3) (and the [test additions that came before it](https://github.com/llvm/llvm-project/pull/169748/commits/a6f444858e04471f263917ea55cb45fa157ad828)).

https://github.com/llvm/llvm-project/pull/169748