[clang] First draft: [Clang] Add constant evaluation support for x86 psadbw (PR #169253)

[Mohsen Rahmati via cfe-commits]

Mohxen wrote:

Hi @RKSimon,

I updated this draft based on your feedback. This now covers the PSADBW intrinsics requested in #188062. This was also split out from #157522.

The patch now handles `psadbw128`, `psadbw256`, and `psadbw512` in both constexpr evaluators, uses `APIntOps::abdu` for the unsigned absolute difference, and keeps the fixed-vector checks as asserts.

I also moved the builtins into existing `Constexpr` builtin groups, marked the public wrappers constexpr, and replaced the separate AST/ByteCode test with `TEST_CONSTEXPR` checks in the existing X86 builtin test files.

Covered wrappers:
`_mm_sad_pu8`, `_mm_sad_epu8`, `_mm256_sad_epu8`, and `_mm512_sad_epu8`.

Local targeted tests pass: `mmx-builtins.c`, `sse2-builtins.c`, `avx2-builtins.c`, and `avx512bw-builtins.c`.

https://github.com/llvm/llvm-project/pull/169253