[Clang] VectorExprEvaluator::VisitCallExpr / InterpretBuiltin - allow AVX subvector shuffles intrinsics to be used in constexpr

[RKSimon]

Split off from #160514 as these are the only ones that require zero handling:

_mm256_permute2f128_pd
_mm256_permute2f128_ps
_mm256_permute2f128_si256
_mm256_permute2x128_si256

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

Split off from #160514 as these are the only ones that require zero handling: ``` _mm256_permute2f128_pd _mm256_permute2f128_ps _mm256_permute2f128_si256 _mm256_permute2x128_si256 ```

[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)

Split off from #160514 as these are the only ones that require zero handling: ``` _mm256_permute2f128_pd _mm256_permute2f128_ps _mm256_permute2f128_si256 _mm256_permute2x128_si256 ```

[AhmedYasser5]

Hello @RKSimon , I'd like to work on this issue. Could you guide me to work on it?

[AhmedYasser5]

Split off from #160514 as these are the only ones that require zero handling:

_mm256_permute2f128_pd
_mm256_permute2f128_ps
_mm256_permute2f128_si256
_mm256_permute2x128_si256

@RKSimon I just noticed that _mm256_permute2x128_si256 (permti256) is for avx2, and it's in a group of its own away from the other 3 permutes. Is it intended to handle it separately at this phase?

[RKSimon]

No, you should be able to handle all of them the same - they use the same immediate