[Headers][X86] Allow MMX/SSE/AVX MOVMSK intrinsics to be used in constexpr

[RKSimon]

_mm_movemask_pi8 _mm_movemask_epi8 _mm256_movemask_epi8
_mm_movemask_ps _mm256_movemask_ps 
_mm_movemask_pd _mm256_movemask_pd

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

``` _mm_movemask_pi8 _mm_movemask_epi8 _mm256_movemask_epi8 _mm_movemask_ps _mm256_movemask_ps _mm_movemask_pd _mm256_movemask_pd ```

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Simon Pilgrim (RKSimon)

``` _mm_movemask_pi8 _mm_movemask_epi8 _mm256_movemask_epi8 _mm_movemask_ps _mm256_movemask_ps _mm_movemask_pd _mm256_movemask_pd ```

[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)

``` _mm_movemask_pi8 _mm_movemask_epi8 _mm256_movemask_epi8 _mm_movemask_ps _mm256_movemask_ps _mm_movemask_pd _mm256_movemask_pd ```

[alina-weber]

Hey,
I would like to work on an issue to get to know LLVM. I picked this one as the header seemed somehow familiar to me. Could I get assigned this issue?
Is there any time pressure to resolve this issues?

[RKSimon]

Hey, I would like to work on an issue to get to know LLVM. I picked this one as the header seemed somehow familiar to me. Could I get assigned this issue? Is there any time pressure to resolve this issues?

No immediate time pressures - we're just trying to get as many of the x86 intrinsics constexpr ready done as possible for the next release at the end of the year.

Its useful to others to track progress if you can instead of just going quiet though (public branch, draft PR etc.).

[alina-weber]

Ok thanks!
I will need some time (should not be more than a month hopefully) to dig into how things work and set everything up, but as soon as there is a draft I will make a PR.
If I don't find the time to do the issue for whatever reason, I will notify you so the issue can be assigned to someone else.