Add [T]::as_simd(_mut)

SIMD-style optimizations are the most common use for `[T]::align_to(_mut)`, but that's `unsafe`.  So these are *safe* wrappers around it, now that we have the `Simd` type available, to make it easier to use.

```rust
impl [T] {
    pub fn as_simd<const LANES: usize>(&self) -> (&[T], &[Simd<T, LANES>], &[T]);
    pub fn as_simd_mut<const LANES: usize>(&mut self) -> (&mut [T], &mut [Simd<T, LANES>], &mut [T]);
}
```

library/core/src/slice/mod.rs

@@ -16,6 +16,8 @@ use crate::option::Option::{None, Some};
 use crate::ptr;
 use crate::result::Result;
 use crate::result::Result::{Err, Ok};
+#[cfg(not(miri))] // Miri does not support all SIMD intrinsics
+use crate::simd::{self, Simd};
 use crate::slice;
 
 #[unstable(
@@ -3434,6 +3436,87 @@ impl<T> [T] {
         }
     }
 
+    /// Split a slice into a prefix, a middle of aligned simd types, and a suffix.
+    ///
+    /// This is a safe wrapper around [`slice::align_to`], so has the same weak
+    /// preconditions as that method.  Notably, you must not assume any particular
+    /// split between the three parts: it's legal for the middle slice to be
+    /// empty even if the input slice is longer than `3 * LANES`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(portable_simd)]
+    ///
+    /// let short = &[1, 2, 3];
+    /// let (prefix, middle, suffix) = short.as_simd::<4>();
+    /// assert_eq!(middle, []); // Not enough elements for anything in the middle
+    ///
+    /// // They might be split in any possible way between prefix and suffix
+    /// let it = prefix.iter().chain(suffix).copied();
+    /// assert_eq!(it.collect::<Vec<_>>(), vec![1, 2, 3]);
+    ///
+    /// fn basic_simd_sum(x: &[f32]) -> f32 {
+    ///     use std::ops::Add;
+    ///     use std::simd::f32x4;
+    ///     let (prefix, middle, suffix) = x.as_simd();
+    ///     let sums = f32x4::from_array([
+    ///         prefix.iter().copied().sum(),
+    ///         0.0,
+    ///         0.0,
+    ///         suffix.iter().copied().sum(),
+    ///     ]);
+    ///     let sums = middle.iter().copied().fold(sums, f32x4::add);
+    ///     sums.horizontal_sum()
+    /// }
+    ///
+    /// let numbers: Vec<f32> = (1..101).map(|x| x as _).collect();
+    /// assert_eq!(basic_simd_sum(&numbers[1..99]), 4949.0);
+    /// ```
+    #[unstable(feature = "portable_simd", issue = "86656")]
+    #[cfg(not(miri))] // Miri does not support all SIMD intrinsics
+    pub fn as_simd<const LANES: usize>(&self) -> (&[T], &[Simd<T, LANES>], &[T])
+    where
+        Simd<T, LANES>: AsRef<[T; LANES]>,
+        T: simd::SimdElement,
+        simd::LaneCount<LANES>: simd::SupportedLaneCount,
+    {
+        // These are expected to always match, as vector types are laid out like
+        // arrays per <https://llvm.org/docs/LangRef.html#vector-type>, but we
+        // might as well double-check since it'll optimize away anyhow.
+        assert_eq!(mem::size_of::<Simd<T, LANES>>(), mem::size_of::<[T; LANES]>());
+
+        // SAFETY: The simd types have the same layout as arrays, just with
+        // potentially-higher alignment, so the de-facto transmutes are sound.
+        unsafe { self.align_to() }
+    }
+
+    /// Split a slice into a prefix, a middle of aligned simd types, and a suffix.
+    ///
+    /// This is a safe wrapper around [`slice::align_to`], so has the same weak
+    /// preconditions as that method.  Notably, you must not assume any particular
+    /// split between the three parts: it's legal for the middle slice to be
+    /// empty even if the input slice is longer than `3 * LANES`.
+    ///
+    /// This is the mutable version of [`slice::as_simd`]; see that for more.
+    #[unstable(feature = "portable_simd", issue = "86656")]
+    #[cfg(not(miri))] // Miri does not support all SIMD intrinsics
+    pub fn as_simd_mut<const LANES: usize>(&mut self) -> (&mut [T], &mut [Simd<T, LANES>], &mut [T])
+    where
+        Simd<T, LANES>: AsMut<[T; LANES]>,
+        T: simd::SimdElement,
+        simd::LaneCount<LANES>: simd::SupportedLaneCount,
+    {
+        // These are expected to always match, as vector types are laid out like
+        // arrays per <https://llvm.org/docs/LangRef.html#vector-type>, but we
+        // might as well double-check since it'll optimize away anyhow.
+        assert_eq!(mem::size_of::<Simd<T, LANES>>(), mem::size_of::<[T; LANES]>());
+
+        // SAFETY: The simd types have the same layout as arrays, just with
+        // potentially-higher alignment, so the de-facto transmutes are sound.
+        unsafe { self.align_to_mut() }
+    }
+
     /// Checks if the elements of this slice are sorted.
     ///
     /// That is, for each element `a` and its following element `b`, `a <= b` must hold. If the