[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 `ppc64le-flang-rhel-clang` running on `ppc64le-flang-rhel-test` while building `llvm` at step 7 "test-build-unified-tree-check-flang-rt".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/157/builds/42472

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

```
Step 7 (test-build-unified-tree-check-flang-rt) failure: 1200 seconds without output running [b'ninja', b'check-flang-rt'], attempting to kill

```

</details>

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