Add alignment parameter to simd_masked_{load,store}

[sayantn]

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.

• cg_llvm
• cg_clif
• 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