[llvm] [LLVM][InstCombine][AArch64] Refactor common SVE intrinsic combines. (PR #126928)

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

The PR deliberately maintains the combines original use of `undef` so the changes are NFC from a testing point of view.  I plan to follow up with a dedicated patch to remove the use of `undef` and am thus ignoring the above "formatting" failure.

https://github.com/llvm/llvm-project/pull/126928