[llvm] [AArch64][SVE] Avoid movprfx by reusing register for _UNDEF pseudos. (PR #166926)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `lld-x86_64-ubuntu-fast` running on `as-builder-4` while building `llvm` at step 2 "checkout".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/33/builds/26184

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 2 (checkout) failure: update (failure)
git version 2.43.0
error: RPC failed; HTTP 500 curl 22 The requested URL returned error: 500
fatal: expected flush after ref listing
error: RPC failed; HTTP 500 curl 22 The requested URL returned error: 500
fatal: expected flush after ref listing

```

</details>

https://github.com/llvm/llvm-project/pull/166926