[llvm] [SimplifyCFG] Replace unreachable switch lookup table holes with poison (PR #94990)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-aarch64-sve-vla` running on `linaro-g3-03` while building `llvm` at step 1 "Checkout lnt".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/17/builds/4896

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

```
Step 1 (Checkout lnt) failure: update (failure)
git version 2.34.1
>From https://github.com/llvm/llvm-lnt
 * branch            HEAD       -> FETCH_HEAD
fatal: Unable to create '/home/tcwg-buildbot/worker/clang-aarch64-sve-vla/test/lnt/.git/index.lock': File exists.

Another git process seems to be running in this repository, e.g.
an editor opened by 'git commit'. Please make sure all processes
are terminated then try again. If it still fails, a git process
may have crashed in this repository earlier:
remove the file manually to continue.
>From https://github.com/llvm/llvm-lnt
 * branch            HEAD       -> FETCH_HEAD
fatal: Unable to create '/home/tcwg-buildbot/worker/clang-aarch64-sve-vla/test/lnt/.git/index.lock': File exists.

Another git process seems to be running in this repository, e.g.
an editor opened by 'git commit'. Please make sure all processes
are terminated then try again. If it still fails, a git process
may have crashed in this repository earlier:
remove the file manually to continue.

```

</details>

https://github.com/llvm/llvm-project/pull/94990