These tend to have special handling in a bunch of places anyway, so the variant helps remember that. And I think it's easier to grok than non-Scalar Aggregates sometimes being `Immediates` (like I got wrong and caused 109992). As a minor bonus, it means we don't need to generate poison LLVM values for them to pass around in `OperandValue::Immediate`s.
This takes a whole 3 lines in `compiler/` since it lowers to `CastKind::Transmute` in MIR *exactly* the same as the existing `intrinsics::transmute` does, it just doesn't have the fancy checking in `hir_typeck`.
Added to enable experimenting with the request in <https://github.com/rust-lang/rust/pull/106281#issuecomment-1496648190> and because the portable-simd folks might be interested for dependently-sized array-vector conversions.
It also simplifies a couple places in `core`.
LLVM can usually optimize these away, but especially for things like transmutes of newtypes it's silly to generate the `alloc`+`store`+`load` at all when it's actually a nop at LLVM level.
Updates `interpret`, `codegen_ssa`, and `codegen_cranelift` to consume the new cast instead of the intrinsic.
Includes `CastTransmute` for custom MIR building, to be able to test the extra UB.
