<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/152315>152315</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Headers][X86] Allow SSE41/AVX2/AVX512F/AVX512BW integer extension intrinsics to be used in constexpr
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          RKSimon
      </td>
    </tr>
</table>

<pre>
    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
</pre>
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