[llvm] [SLP] Fix cost estimation of external uses with wrong VF (PR #148185)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `openmp-gcc-x86_64-linux-debian` running on `gribozavr4` while building `llvm` at step 2 "checkout".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/70/builds/10663

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

```
Step 2 (checkout) failure: update (failure)
Upon execvpe b'git' [b'git', b'--version'] in environment id 140389109472512
:Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/twisted/internet/process.py", line 405, in _fork
    self._execChild(path, uid, gid, executable, args,
  File "/usr/lib/python3/dist-packages/twisted/internet/process.py", line 484, in _execChild
    os.execvpe(executable, args, environment)
  File "/usr/lib/python3.9/os.py", line 583, in execvpe
    _execvpe(file, args, env)
  File "/usr/lib/python3.9/os.py", line 616, in _execvpe
    raise last_exc
  File "/usr/lib/python3.9/os.py", line 607, in _execvpe
    exec_func(fullname, *argrest)
FileNotFoundError: [Errno 2] No such file or directory: b'/bin/git'

```

</details>

https://github.com/llvm/llvm-project/pull/148185