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

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

fixes https://github.com/rust-lang/rust/issues/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`

compiler/rustc_span/src/symbol.rs

@@ -1913,6 +1913,7 @@ symbols! {
         simd_eq,
         simd_expose_provenance,
         simd_extract,
+        simd_extract_dyn,
         simd_fabs,
         simd_fcos,
         simd_fexp,
@@ -1931,6 +1932,7 @@ symbols! {
         simd_ge,
         simd_gt,
         simd_insert,
+        simd_insert_dyn,
         simd_le,
         simd_lt,
         simd_masked_load,