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Add alignment parameter to `simd_masked_{load,store}`
This PR adds an alignment parameter in `simd_masked_load` and `simd_masked_store`, in the form of a const-generic enum `core::intrinsics::simd::SimdAlign`. This represents the alignment of the `ptr` argument in these intrinsics as follows
- `SimdAlign::Unaligned` - `ptr` is unaligned/1-byte aligned
- `SimdAlign::Element` - `ptr` is aligned to the element type of the SIMD vector (default behavior in the old signature)
- `SimdAlign::Vector` - `ptr` is aligned to the SIMD vector type
The main motive for this is stdarch - most vector loads are either fully aligned (to the vector size) or unaligned (byte-aligned), so the previous signature doesn't cut it.
Now, stdarch will mostly use `SimdAlign::Unaligned` and `SimdAlign::Vector`, whereas portable-simd will use `SimdAlign::Element`.
- [x] `cg_llvm`
- [x] `cg_clif`
- [x] `miri`/`const_eval`
## Alternatives
Using a const-generic/"const" `u32` parameter as alignment (and we error during codegen if this argument is not a power of two). This, although more flexible than this, has a few drawbacks
- If we use an const-generic argument, then portable-simd somehow needs to pass `align_of::<T>()` as the alignment, which isn't possible without GCE
- "const" function parameters are just an ugly hack, and a pain to deal with in non-LLVM backends
We can remedy the problem with the const-generic `u32` parameter by adding a special rule for the element alignment case (e.g. `0` can mean "use the alignment of the element type), but I feel like this is not as expressive as the enum approach, although I am open to suggestions
cc `@workingjubilee` `@RalfJung` `@BoxyUwU`
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.