[Headers][X86] Allow SSE41/AVX2/AVX512F/AVX512BW integer extension intrinsics to be used in constexpr

[RKSimon]

All of these intrinsics wrap __builtin_convertvector/__builtin_shufflevector which can already be used in constexpr

_mm_cvtepi8_epi16   _mm_cvtepu8_epi16
_mm_cvtepi8_epi32   _mm_cvtepu8_epi32
_mm_cvtepi8_epi64   _mm_cvtepu8_epi64
_mm_cvtepi16_epi32  _mm_cvtepu16_epi32
_mm_cvtepi16_epi64  _mm_cvtepu16_epi64
_mm_cvtepi32_epi64  _mm_cvtepu32_epi64

_mm256_cvtepi8_epi16   _mm256_cvtepu8_epi16
_mm256_cvtepi8_epi32   _mm256_cvtepu8_epi32
_mm256_cvtepi8_epi64   _mm256_cvtepu8_epi64
_mm256_cvtepi16_epi32  _mm256_cvtepu16_epi32
_mm256_cvtepi16_epi64  _mm256_cvtepu16_epi64
_mm256_cvtepi32_epi64  _mm256_cvtepu32_epi64

_mm512_cvtepi8_epi16   _mm512_cvtepu8_epi16
_mm512_cvtepi8_epi32   _mm512_cvtepu8_epi32
_mm512_cvtepi8_epi64   _mm512_cvtepu8_epi64
_mm512_cvtepi16_epi32  _mm512_cvtepu16_epi32
_mm512_cvtepi16_epi64  _mm512_cvtepu16_epi64
_mm512_cvtepi32_epi64  _mm512_cvtepu32_epi64

(ignore the AVX512 mask/maskz variants for now)

You can use #152296 for reference

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

All of these intrinsics wrap __builtin_convertvector/__builtin_shufflevector which can already be used in constexpr ``` _mm_cvtepi8_epi16 _mm_cvtepu8_epi16 _mm_cvtepi8_epi32 _mm_cvtepu8_epi32 _mm_cvtepi8_epi64 _mm_cvtepu8_epi64 _mm_cvtepi16_epi32 _mm_cvtepu16_epi32 _mm_cvtepi16_epi64 _mm_cvtepu16_epi64 _mm_cvtepi32_epi64 _mm_cvtepu32_epi64

_mm256_cvtepi8_epi16 _mm256_cvtepu8_epi16
_mm256_cvtepi8_epi32 _mm256_cvtepu8_epi32
_mm256_cvtepi8_epi64 _mm256_cvtepu8_epi64
_mm256_cvtepi16_epi32 _mm256_cvtepu16_epi32
_mm256_cvtepi16_epi64 _mm256_cvtepu16_epi64
_mm256_cvtepi32_epi64 _mm256_cvtepu32_epi64

_mm512_cvtepi8_epi16 _mm512_cvtepu8_epi16
_mm512_cvtepi8_epi32 _mm512_cvtepu8_epi32
_mm512_cvtepi8_epi64 _mm512_cvtepu8_epi64
_mm512_cvtepi16_epi32 _mm512_cvtepu16_epi32
_mm512_cvtepi16_epi64 _mm512_cvtepu16_epi64
_mm512_cvtepi32_epi64 _mm512_cvtepu32_epi64

(ignore the AVX512 mask/maskz variants for now)

You can use #<!-- -->152288 for reference
</details>

[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)

All of these intrinsics wrap __builtin_convertvector/__builtin_shufflevector which can already be used in constexpr ``` _mm_cvtepi8_epi16 _mm_cvtepu8_epi16 _mm_cvtepi8_epi32 _mm_cvtepu8_epi32 _mm_cvtepi8_epi64 _mm_cvtepu8_epi64 _mm_cvtepi16_epi32 _mm_cvtepu16_epi32 _mm_cvtepi16_epi64 _mm_cvtepu16_epi64 _mm_cvtepi32_epi64 _mm_cvtepu32_epi64

_mm256_cvtepi8_epi16 _mm256_cvtepu8_epi16
_mm256_cvtepi8_epi32 _mm256_cvtepu8_epi32
_mm256_cvtepi8_epi64 _mm256_cvtepu8_epi64
_mm256_cvtepi16_epi32 _mm256_cvtepu16_epi32
_mm256_cvtepi16_epi64 _mm256_cvtepu16_epi64
_mm256_cvtepi32_epi64 _mm256_cvtepu32_epi64

_mm512_cvtepi8_epi16 _mm512_cvtepu8_epi16
_mm512_cvtepi8_epi32 _mm512_cvtepu8_epi32
_mm512_cvtepi8_epi64 _mm512_cvtepu8_epi64
_mm512_cvtepi16_epi32 _mm512_cvtepu16_epi32
_mm512_cvtepi16_epi64 _mm512_cvtepu16_epi64
_mm512_cvtepi32_epi64 _mm512_cvtepu32_epi64

(ignore the AVX512 mask/maskz variants for now)

You can use #<!-- -->152288 for reference
</details>

[woruyu]

@RKSimon I'd like to work on this! Can you assign it to me?

[el-ev]

@woruyu assigned.