Allow vector zero padding intrinsics to be used in constexpr

[eayus]

Fix #156346 by marking intrinsics as constexpr. A test has been added for each intrinsic.

The following instrinsics have been modified:

_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

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[eayus]

Pinging @RKSimon as I am unable to manually add reviewers.

[github-actions]

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[RKSimon]

@eayus are you able to fix your public github address please?

[RKSimon]

LGTM - cheers

[eayus]

@RKSimon I've changed my account settings to make my email address public, though I'm not sure whether that will retroactively fix the problem?

[RKSimon]

@RKSimon I've changed my account settings to make my email address public, though I'm not sure whether that will retroactively fix the problem?

Yes that seems to have worked - auto-merge now reports your proton address. Thanks.

[github-actions]

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