[Clang] Add __builtin_elementwise_ctlz/cttz builtin intrinsics

[RKSimon]

Implement vector elementwise leading/trailing zero count intrinsics with constexpr support - references #153113 + #153572

I know at the very least AVX512 / NEON both correctly handle the zero input case so I don't believe we need the "is_zero_poison" handling like we do for the llvm intrinsic.

[llvmbot]

Hi!

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[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Simon Pilgrim (RKSimon)

Implement vector elementwise leading/trailing zero count intrinsics with constexpr support - references #153113 + #153572

I know a the very least AVX512 / NEON both correctly handle the zero input case so I don't believe we need the "is_zero_poison" handling like we do for the llvm intrinsic.

[llvmbot]

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

Author: Simon Pilgrim (RKSimon)

Implement vector elementwise leading/trailing zero count intrinsics with constexpr support - references #153113 + #153572

I know a the very least AVX512 / NEON both correctly handle the zero input case so I don't believe we need the "is_zero_poison" handling like we do for the llvm intrinsic.

[tbaederr]

There's #131995 already