Rollup merge of #137447 - folkertdev:simd-extract-insert-dyn, r=scottmcm

add `core::intrinsics::simd::{simd_extract_dyn, simd_insert_dyn}`

fixes rust-lang/rust#137372

adds `core::intrinsics::simd::{simd_extract_dyn, simd_insert_dyn}`, which contrary to their non-dyn counterparts allow a non-const index. Many platforms (but notably not x86_64 or aarch64) have dedicated instructions for this operation, which stdarch can emit with this change.

Future work is to also make the `Index` operation on the `Simd` type emit this operation, but the intrinsic can't be used directly. We'll need some MIR shenanigans for that.

r? `@ghost`

Author: Zalathar