[Headers][X86] Allow SLLDQ/SRLDQ byte shift intrinsics to be used in constexpr

[RKSimon]

The SLLDQ/SRLDQ byte shift intrinsics can be used in constant expression

_mm_slli_si128 _mm256_slli_si256
_mm_srli_si128 _mm256_srli_si256

These will require handling inside VectorExprEvaluator::VisitCallExpr / InterpretBuiltin

NOTE: Ensure you handle out of bounds correctly

[kimyounhoex1]

Hi, I'd like to take this issue as my first LLVM contribution.

Thanks!

[NagrajMG]

I’ll handle the SLLDQ/SRLDQ intrinsic support implementation.

[RKSimon]

@NagrajMG More constexpr issues coming soon if there's nothing else in the list you want to do.

[kimyounhoex1]

@RKSimon @NagrajMG Thanks for assigning me this issue!
I’ll start looking into the code and update here with my progress.

[RKSimon]

Thanks - feel free to make a public branch / draft PR to show progress and if you're after any early feedback.