[clang] [CIR] Implement handling for VectorType with size 3 (PR #161232)

[Bruno Cardoso Lopes via cfe-commits]

bcardosolopes wrote:

> As for what we do in CIR, I was thinking maybe we could initially annotate the vector-3 loads and stores with some attribute indicating that a vector-4 load and store with a poison element is permitted/expected.

I'd leave the attribute as last resort, we can easily identify by the combo "target + vector size".

> If a similar extension were done in LLVM IR, this would lower naturally to a masked load/store on targets that support that, but assuming there will at least be a time before that happens in LLVM IR ...

If you can get an attribute in LLVMIR I guess it makes sense to have that in CIR as well.

https://github.com/llvm/llvm-project/pull/161232