[clang] [Clang] Allow VDBPSADBW intrinsics in constexpr (PR #188887)

[Pierluigi Lenoci via cfe-commits]

pierluigilenoci wrote:

@RKSimon Added mask/maskz constexpr test coverage for all requested variants:

- `_mm256_mask_dbsad_epu8` (mask=0x5555, passthrough odd elements from source)
- `_mm256_maskz_dbsad_epu8` (mask=0xAAAA, zero even elements)
- `_mm512_mask_dbsad_epu8` (mask=0x55555555, passthrough odd elements)
- `_mm512_maskz_dbsad_epu8` (mask=0xAAAAAAAA, zero even elements)

These follow the same pattern as the existing 128-bit mask/maskz tests. CI is green. Let me know if the coverage looks sufficient or if you'd like any adjustments.

https://github.com/llvm/llvm-project/pull/188887