Compiler error at `Simd::splat`: usage of `intrinsics::const_eval_select`

[AquaEBM]

The latest nightly compiler gives the following error when compiling the core_simd crate from this repository (not core::simd):

error: const function that might be (indirectly) exposed to stable cannot use `#[feature(const_eval_select)]`
   --> portable-simd/crates/core_simd/src/vector.rs:171:9
    |
171 |         core::intrinsics::const_eval_select((value,), splat_const, splat_rt)
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
help: if the function is not (yet) meant to be exposed to stable, add `#[rustc_const_unstable]` (this is what you probably want to do)
    |
147 +     #[rustc_const_unstable(feature = "...", issue = "...")]
148 |     pub const fn splat(value: T) -> Self {
    |
help: otherwise, as a last resort `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks (this requires team approval)
    |
147 +     #[rustc_allow_const_fn_unstable(const_eval_select)]
148 |     pub const fn splat(value: T) -> Self {
    |

A quick blame suggests that this is caused by #417

Possible fixes (some already suggested by the compiler):

• Using #[rustc_const_unstable(feature = "...", issue = "...")] (Which feature and tracking issue would be appropriate here?)
• Using #[rustc_allow_const_fn_unstable(const_eval_select)]
• Reverting Make splat const fn #417 completely

[calebzulawski]

I think it might be reasonable to use the unstable attribute with this feature and tracking issue: https://github.com/rust-lang/rust/blob/826b673412d2f579e7865b6f26bc9771c6d3b097/library/core/src/lib.rs#L437

Using the unstable attribute gives us the reminder to reevaluate this in the future when we are ready to stabilize.