[llvm] [AArch64][SVE] Combine UXT[BHW] intrinsics to AND. (PR #137956)

[Ricardo Jesus via llvm-commits]

rj-jesus wrote:

> FYI: I have been investigating whether we can replace these uses of `undef` with `poison` but it's not entirely straightforward. I have a plan but there are several pieces that need completing before I can change clang because otherwise we're likely to see regression in generated code.

Thanks for the heads-up! I can imagine it won't be easy to make sure everything still plays nicely.
As things stand, I don't think there's another way of testing ACLE's ``don't care'' merging other than with `undef` values as in this patch, right? 

https://github.com/llvm/llvm-project/pull/137956