[llvm] [Vectorize] Remove a redundant declaration (NFC) (PR #167188)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-with-thin-lto-ubuntu` running on `as-worker-92` while building `llvm` at step 27 "compare-compilers".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/127/builds/5242

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

```
Step 27 (compare-compilers) failure: 'diff -q ...' (failure)
Files install/stage3/bin/clang and install/stage4/bin/clang differ

```

</details>

https://github.com/llvm/llvm-project/pull/167188