[llvm] [RISCV] Combine vslide{up,down} x, poison -> x (PR #169013)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `publish-sphinx-docs` running on `as-worker-4` while building `llvm` at step 6 "Publish docs-llvm-html".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/45/builds/18477

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

```
Step 6 (Publish docs-llvm-html) failure: 'rsync -vrl ...' (failure)
Connection closed by 54.67.122.174 port 22
rsync: connection unexpectedly closed (0 bytes received so far) [sender]
rsync error: unexplained error (code 255) at io.c(232) [sender=3.2.7]
Step 11 (Publish docs-flang-html) failure: 'rsync -vrl ...' (failure)
Connection closed by 54.67.122.174 port 22
rsync: connection unexpectedly closed (0 bytes received so far) [sender]
rsync error: unexplained error (code 255) at io.c(232) [sender=3.2.7]

```

</details>

https://github.com/llvm/llvm-project/pull/169013