[X86][Clang] VectorExprEvaluator::VisitCallExpr / InterpretBuiltin - allow AVX512 kmov intrinsics to be used in constexp

[RKSimon]

These wrap the __builtin_ia32_kmov* builtins, which are just NOPs used to create mask predicates

unsigned int _cvtmask16_u32 (__mmask16 a)
unsigned int _cvtmask32_u32 (__mmask32 a)
unsigned __int64 _cvtmask64_u64 (__mmask64 a)
unsigned int _cvtmask8_u32 (__mmask8 a)

__mmask64 _cvtu64_mask64 (unsigned __int64 a)
__mmask16 _cvtu32_mask16 (unsigned int a)
__mmask32 _cvtu32_mask32 (unsigned int a)
__mmask8 _cvtu32_mask8 (unsigned int a)

We still need very basic test coverage just to prove they work in constexpr

[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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3. Fix the issue locally.
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6. Run git clang-format HEAD~1 to format your changes.
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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)

These wrap the __builtin_ia32_kmov* builtins, which are just NOPs used to create mask predicates ``` unsigned int _cvtmask16_u32 (__mmask16 a) unsigned int _cvtmask32_u32 (__mmask32 a) unsigned __int64 _cvtmask64_u64 (__mmask64 a) unsigned int _cvtmask8_u32 (__mmask8 a)

__mmask64 _cvtu64_mask64 (unsigned __int64 a)
__mmask16 _cvtu32_mask16 (unsigned int a)
__mmask32 _cvtu32_mask32 (unsigned int a)
__mmask8 _cvtu32_mask8 (unsigned int a)

We still need very basic test coverage just to prove they work in constexpr
</details>

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

These wrap the __builtin_ia32_kmov* builtins, which are just NOPs used to create mask predicates ``` unsigned int _cvtmask16_u32 (__mmask16 a) unsigned int _cvtmask32_u32 (__mmask32 a) unsigned __int64 _cvtmask64_u64 (__mmask64 a) unsigned int _cvtmask8_u32 (__mmask8 a)

__mmask64 _cvtu64_mask64 (unsigned __int64 a)
__mmask16 _cvtu32_mask16 (unsigned int a)
__mmask32 _cvtu32_mask32 (unsigned int a)
__mmask8 _cvtu32_mask8 (unsigned int a)

We still need very basic test coverage just to prove they work in constexpr
</details>

[sivk-kex]

Hello, I would like to work on this issue.

[sivk-kex]

@RKSimon would it be possible to delay this till 24th as I have exams?