Auto merge of #90821 - scottmcm:new-slice-reverse, r=Mark-Simulacrum

MIRI says `reverse` is UB, so replace it with something LLVM can vectorize

For small types with padding, the current implementation is UB because it does integer operations on uninit values.
```
error: Undefined Behavior: using uninitialized data, but this operation requires initialized memory
   --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/num/mod.rs:836:5
    |
836 | /     uint_impl! { u32, u32, i32, 32, 4294967295, 8, "0x10000b3", "0xb301", "0x12345678",
837 | |     "0x78563412", "0x1e6a2c48", "[0x78, 0x56, 0x34, 0x12]", "[0x12, 0x34, 0x56, 0x78]", "", "" }
    | |________________________________________________________________________________________________^ using uninitialized data, but this operation requires initialized memory
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

    = note: inside `core::num::<impl u32>::rotate_left` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/num/uint_macros.rs:211:13
    = note: inside `core::slice::<impl [Foo]>::reverse` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/slice/mod.rs:701:58
```
<https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=340739f22ca5b457e1da6f361768edc6>

But LLVM has gotten smarter since I wrote the previous implementation in 2017, so this PR removes all the manual magic and just writes it in such a way that LLVM will vectorize.  This code is much simpler and has very little `unsafe`, and is actually faster to boot!

If you're curious to see the codegen: <https://rust.godbolt.org/z/Pcn13Y9E3>

Before:
```
running 7 tests
test slice::reverse_simd_f64x4                           ... bench:      17,940 ns/iter (+/- 481) = 58448 MB/s
test slice::reverse_u128                                 ... bench:      17,758 ns/iter (+/- 205) = 59048 MB/s
test slice::reverse_u16                                  ... bench:     158,234 ns/iter (+/- 6,876) = 6626 MB/s
test slice::reverse_u32                                  ... bench:      62,047 ns/iter (+/- 1,117) = 16899 MB/s
test slice::reverse_u64                                  ... bench:      31,582 ns/iter (+/- 552) = 33201 MB/s
test slice::reverse_u8                                   ... bench:      81,253 ns/iter (+/- 1,510) = 12905 MB/s
test slice::reverse_u8x3                                 ... bench:     270,615 ns/iter (+/- 11,463) = 3874 MB/s
```

After:
```
running 7 tests
test slice::reverse_simd_f64x4                           ... bench:      17,731 ns/iter (+/- 306) = 59137 MB/s
test slice::reverse_u128                                 ... bench:      17,919 ns/iter (+/- 239) = 58517 MB/s
test slice::reverse_u16                                  ... bench:      43,160 ns/iter (+/- 607) = 24295 MB/s
test slice::reverse_u32                                  ... bench:      21,065 ns/iter (+/- 371) = 49778 MB/s
test slice::reverse_u64                                  ... bench:      21,118 ns/iter (+/- 482) = 49653 MB/s
test slice::reverse_u8                                   ... bench:      76,878 ns/iter (+/- 1,688) = 13639 MB/s
test slice::reverse_u8x3                                 ... bench:     264,723 ns/iter (+/- 5,544) = 3961 MB/s
```

Those are the existing benches, <https://github.com/rust-lang/rust/blob/14a2fd640e0df9ee8cc1e04280b0c3aff93c42da/library/alloc/benches/slice.rs#L322-L346>

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