[Headers][X86] Allow MMX/SSE2/AVX2/AVX512BW integer saturated arithmetic intrinsics to be used in constexpr

[RKSimon]

All of these intrinsics wrap generic __builtin_elementwise_add_sat/sub_sat which can already be used in constexpr

_mm_adds_pi8 _mm_adds_pi16
_mm_subs_pi8 _mm_subs_pi16

_mm_adds_epi8 _mm_adds_epi16
_mm_subs_epi8 _mm_subs_epi16

_mm256_adds_epi8 _mm256_adds_epi16
_mm256_subs_epi8 _mm256_subs_epi16

_mm512_adds_epi8 _mm512_adds_epi16
_mm512_subs_epi8 _mm512_subs_epi16

(ignore the AVX512 mask/maskz variants for now)

You can use #152296 for reference - just make sure some of the constants actually saturate!

[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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[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Simon Pilgrim (RKSimon)

All of these intrinsics wrap generic __builtin_elementwise_add_sat/sub_sat which can already be used in constexpr ``` _mm_adds_epi8 _mm_adds_epi16 _mm_subs_epi8 _mm_subs_epi16

_mm256_adds_epi8 _mm256_adds_epi16
_mm256_subs_epi8 _mm256_subs_epi16

_mm512_adds_epi8 _mm512_adds_epi16
_mm512_subs_epi8 _mm512_subs_epi16

(ignore the AVX512 mask/maskz variants for now)

You can use https://github.com/llvm/llvm-project/pull/152296 for reference - just make sure some of the constants actually saturate!
</details>

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

All of these intrinsics wrap generic __builtin_elementwise_add_sat/sub_sat which can already be used in constexpr ``` _mm_adds_epi8 _mm_adds_epi16 _mm_subs_epi8 _mm_subs_epi16

_mm256_adds_epi8 _mm256_adds_epi16
_mm256_subs_epi8 _mm256_subs_epi16

_mm512_adds_epi8 _mm512_adds_epi16
_mm512_subs_epi8 _mm512_subs_epi16

(ignore the AVX512 mask/maskz variants for now)

You can use https://github.com/llvm/llvm-project/pull/152296 for reference - just make sure some of the constants actually saturate!
</details>

[ningxinr]

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