[Headers][X86] Allow F16C f16 -> f32 intrinsics to be used in constexpr

[RKSimon]

Similar to #152315 - generic casts/__builtin_convertvector allows us to make these constexpr with suitable testing

_cvtsh_ss
_mm_cvtph_ps
_mm256_cvtph_ps

[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-backend-x86

Author: Simon Pilgrim (RKSimon)

Similar to #152315 - generic casts/__builtin_convertvector allows us to make these constexpr with suitable testing ``` _cvtsh_ss _mm_cvtph_ps _mm256_cvtph_ps ```

[llvmbot]

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

Author: Simon Pilgrim (RKSimon)

Similar to #152315 - generic casts/__builtin_convertvector allows us to make these constexpr with suitable testing ``` _cvtsh_ss _mm_cvtph_ps _mm256_cvtph_ps ```

[abhinavp5]

Hey, I'd like to work on this, could you please assign me?