[Headers][X86] Allow AVX1 fixed extraction intrinsics to be used in constexpr #154555

[RKSimon]

constexpr support for these helpers seem to have been missed in earlier patches:

_mm256_cvtsd_f64
_mm256_cvtsi256_si32
_mm256_cvtss_f32

[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.
2. Leave a comment indicating that you are working on the issue, or just create a pull request after following the steps below. Mention this issue in the description of the pull request.
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.
5. Create a Git commit.
6. Run git clang-format HEAD~1 to format your changes.
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: Simon Pilgrim (RKSimon)

constexpr support for these helpers seem to have been missed in earlier patches: ``` _mm256_cvtsd_f64 _mm256_cvtsi256_si32 _mm256_cvtss_f32 ```

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

constexpr support for these helpers seem to have been missed in earlier patches: ``` _mm256_cvtsd_f64 _mm256_cvtsi256_si32 _mm256_cvtss_f32 ```

[ckoparkar]

I can add these.