[Headers][X86] Allow vector zero padding intrinsics to be used in constexpr

[RKSimon]

_mm256_zextpd128_pd256
_mm512_zextpd128_pd512
_mm512_zextpd256_pd512
_mm256_zextph128_ph256
_mm512_zextph128_ph512
_mm512_zextph256_ph512
_mm256_zextps128_ps256
_mm512_zextps128_ps512
_mm512_zextps256_ps512
_mm256_zextsi128_si256
_mm512_zextsi128_si512
_mm512_zextsi256_si512

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

``` _mm256_zextpd128_pd256 _mm512_zextpd128_pd512 _mm512_zextpd256_pd512 _mm256_zextph128_ph256 _mm512_zextph128_ph512 _mm512_zextph256_ph512 _mm256_zextps128_ps256 _mm512_zextps128_ps512 _mm512_zextps256_ps512 _mm256_zextsi128_si256 _mm512_zextsi128_si512 _mm512_zextsi256_si512 ```

[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:

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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)

``` _mm256_zextpd128_pd256 _mm512_zextpd128_pd512 _mm512_zextpd256_pd512 _mm256_zextph128_ph256 _mm512_zextph128_ph512 _mm512_zextph256_ph512 _mm256_zextps128_ps256 _mm512_zextps128_ps512 _mm512_zextps256_ps512 _mm256_zextsi128_si256 _mm512_zextsi128_si512 _mm512_zextsi256_si512 ```

[eayus]

May I be assigned to this issue please?