[llvm] [NFC][TableGen][X86] Use StringSwitch to map from string -> enum (PR #139929)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-cmake-x86_64-avx512-linux` running on `avx512-intel64` while building `llvm` at step 13 "setup lit".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/133/builds/16189

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

```
Step 13 (setup lit) failure: '/localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/test/sandbox/bin/python /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/test/lnt/setup.py ...' (failure)
...
writing top-level names to LNT.egg-info/top_level.txt
reading manifest file 'LNT.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
no previously-included directories found matching 'docs/_build'
no previously-included directories found matching 'tests/*/Output'
no previously-included directories found matching 'tests/*/*/Output'
adding license file 'LICENSE.TXT'
writing manifest file 'LNT.egg-info/SOURCES.txt'
running build_ext
copying build/lib.linux-x86_64-cpython-39/lnt/testing/profile/cPerf.cpython-39-x86_64-linux-gnu.so -> lnt/testing/profile
Creating /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/test/sandbox/lib64/python3.9/site-packages/LNT.egg-link (link to .)
Adding LNT 0.4.2.dev0 to easy-install.pth file
Installing lnt script to /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/test/sandbox/bin

Installed /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/test/lnt
Processing dependencies for LNT==0.4.2.dev0
Searching for certifi
Reading https://pypi.org/simple/certifi/
Downloading https://files.pythonhosted.org/packages/4a/7e/3db2bd1b1f9e95f7cddca6d6e75e2f2bd9f51b1246e546d88addca0106bd/certifi-2025.4.26-py3-none-any.whl#sha256=30350364dfe371162649852c63336a15c70c6510c2ad5015b21c2345311805f3
Best match: certifi 2025.4.26
Processing certifi-2025.4.26-py3-none-any.whl
Installing certifi-2025.4.26-py3-none-any.whl to /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/test/sandbox/lib64/python3.9/site-packages
Adding certifi 2025.4.26 to easy-install.pth file

Installed /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/test/sandbox/lib64/python3.9/site-packages/certifi-2025.4.26-py3.9.egg
Searching for lit==0.11.1
Reading https://pypi.org/simple/lit/
Downloading https://files.pythonhosted.org/packages/c8/20/a1c3f83acd15874d953b26f87d94d92bde23d614a711c9e75f60744df2f3/lit-0.11.1-py3-none-any.whl#sha256=4fa0cafcedf9e278d4ffa69bfa1a0482db41fdb1d055c8b9fc29501511abeb16
Best match: lit 0.11.1
Processing lit-0.11.1-py3-none-any.whl
Installing lit-0.11.1-py3-none-any.whl to /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/test/sandbox/lib64/python3.9/site-packages
Adding lit 0.11.1 to easy-install.pth file
detected new path './certifi-2025.4.26-py3.9.egg'
Installing lit script to /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/test/sandbox/bin

Installed /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/test/sandbox/lib64/python3.9/site-packages/lit-0.11.1-py3.9.egg
Searching for requests
Reading https://pypi.org/simple/requests/
Downloading https://files.pythonhosted.org/packages/f9/9b/335f9764261e915ed497fcdeb11df5dfd6f7bf257d4a6a2a686d80da4d54/requests-2.32.3-py3-none-any.whl#sha256=70761cfe03c773ceb22aa2f671b4757976145175cdfca038c02654d061d6dcc6
Best match: requests 2.32.3
Processing requests-2.32.3-py3-none-any.whl
Installing requests-2.32.3-py3-none-any.whl to /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/test/sandbox/lib64/python3.9/site-packages
Adding requests 2.32.3 to easy-install.pth file
detected new path './lit-0.11.1-py3.9.egg'

Installed /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/test/sandbox/lib64/python3.9/site-packages/requests-2.32.3-py3.9.egg
Searching for pyyaml==5.1.2
Reading https://pypi.org/simple/pyyaml/
Downloading https://files.pythonhosted.org/packages/e3/e8/b3212641ee2718d556df0f23f78de8303f068fe29cdaa7a91018849582fe/PyYAML-5.1.2.tar.gz#sha256=01adf0b6c6f61bd11af6e10ca52b7d4057dd0be0343eb9283c878cf3af56aee4
error: The read operation timed out

```

</details>

https://github.com/llvm/llvm-project/pull/139929