[llvm] [LV] Allow scalable VFs in `-force-vector-width` (and use in tests) (PR #204953)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-aarch64-quick` running on `linaro-clang-aarch64-quick` while building `llvm` at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/65/builds/36168

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

```
Step 5 (ninja check 1) failure: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
...
[169/173] Building CXX object unittests/Transforms/Vectorize/CMakeFiles/VectorizeTests.dir/VPlanTest.cpp.o
[170/173] Linking CXX executable unittests/Transforms/Vectorize/VectorizeTests
[171/173] Building CXX object unittests/SandboxIR/CMakeFiles/SandboxIRTests.dir/SandboxIRTest.cpp.o
[172/173] Linking CXX executable unittests/SandboxIR/SandboxIRTests
[172/173] Running all regression tests
llvm-lit: /home/tcwg-buildbot/worker/clang-aarch64-quick/llvm/llvm/utils/lit/lit/llvm/config.py:569: note: using split-file: /home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/bin/split-file
llvm-lit: /home/tcwg-buildbot/worker/clang-aarch64-quick/llvm/llvm/utils/lit/lit/llvm/config.py:569: note: using yaml2obj: /home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/bin/yaml2obj
llvm-lit: /home/tcwg-buildbot/worker/clang-aarch64-quick/llvm/llvm/utils/lit/lit/llvm/config.py:569: note: using llvm-objcopy: /home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/bin/llvm-objcopy
llvm-lit: /home/tcwg-buildbot/worker/clang-aarch64-quick/llvm/llvm/utils/lit/lit/llvm/config.py:569: note: using clang: /home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/bin/clang
-- Testing: 95953 of 95956 tests, 256 workers --
command timed out: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1264.082436

```

</details>

https://github.com/llvm/llvm-project/pull/204953