[LangRef] Vector intrinsics after partial reductions are not organised into categories

[MacDue]

See image above. Only the first two intrinsic are partial reductions; the rest currently don't have categories.

These intrinsics should be sorted into appropriate categories.

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. Check that no other contributor is working on this issue. If someone is assigned to the issue or claimed to be working on it, ping the person. After one week without a response, the assignee may be changed.
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3. Fix the issue locally.
4. Run the test suite locally. Remember that the subdirectories under test/ create fine-grained testing targets, so you can e.g. use make check-clang-ast to only run Clang's AST tests.
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7. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation. Mention this issue in the description of the pull request.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Benjamin Maxwell (MacDue)

<img width="507" height="358" alt="Image" src="https://github.com/user-attachments/assets/a9074579-577f-4608-a312-b718a0d5a04f" />

See image above. Only the first two intrinsic are partial reductions; the rest currently don't have categories.

These intrinsics should be sorted into appropriate categories.