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

[Andy Kaylor via cfe-commits]

https://github.com/andykaylor commented:

This really feels like an optimization that has no business happening in codegen. 

@bcardosolopes , @erichkeane What do you think?

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