abi: use PassMode::Direct even for data types that can be passed as scalar pairs.

[eddyb]

Ideally this change would apply only to #[rust_gpu::vector::v1] types (as they're the ones which can end up being passed as pairs when they have only 2 components, e.g. glam::{Vec2,IVec2,UVec2} etc.), but checking that from a query override (which is where this logic is right now) isn't exactly trivial.

However, I don't expect doing this across the board (i.e. to all structs and enums that would've gotten passed as two scalar args) to cause any issues, not even performance ones (at least on reasonable drivers).

Motivated by (and unlocks):

• abi: Make glam types abi layout match CPU #380

More specifically, it should remove the dependency on this other PR:

• Support scalar pair ABI #381
  (still want to land that, but it makes more sense as an diagnostic UX improvement, while semantically IMO we should not be decomposing and recomposing vector types just because we're not using #[repr(simd)] anymore)

[Firestar99]

Looks good!

I tried rebasing #381 locally and ran into some compiletest disassembly changes, caused both by this PR and the "Erase explicit layout decorations" PR, but all trivial changes. (Next to me forgetting to clear certain target folders)