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

[via llvm-commits]

valadaptive wrote:

I've updated the PR description to remove references to the cleanup PR that didn't work out.

I've managed to answer my own question about the semantics of `undef` vs. `poison`. I'll probably submit a PR clarifying the language reference, but [an Alive2 proof](https://alive2.llvm.org/ce/z/meA_DR) says it *is* correct for an `undef` shuffle index to produce a `poison` element.

As such, this PR is ready to merge; everything is all set on my end.

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