Rollup merge of rust-lang#137551 - folkertdev:import-simd-intrinsics, r=RalfJung

import `simd_` intrinsics

In most cases, we can import the simd intrinsics rather than redeclare them. Apparently, most of these tests were written before `std::intrinsics::simd` existed.

There are a couple of exceptions where we can't yet import:

- the intrinsics are not declared as `const fn` in the standard library, causing issues in the `const-eval` tests
- the `simd_shuffle_generic` function is not exposed from `std::intrinsics`
- the `simd_fpow` and `simd_fpowi` functions are not exposed from `std::intrinsics` (removed in rust-lang#137595)
- some tests use `no_core`, and therefore cannot use `std::intrinsics`

r? ```@RalfJung```

cc ```@workingjubilee``` do you have context on why some intrinsics are not exposed?

Author: jieyouxu

library/core/src/intrinsics/simd.rs

@@ -11,7 +11,7 @@
 /// `idx` must be in-bounds of the vector.
 #[rustc_intrinsic]
 #[rustc_nounwind]
-pub unsafe fn simd_insert<T, U>(_x: T, _idx: u32, _val: U) -> T;
+pub const unsafe fn simd_insert<T, U>(_x: T, _idx: u32, _val: U) -> T;
 
 /// Extracts an element from a vector.
 ///
@@ -22,7 +22,7 @@ pub unsafe fn simd_insert<T, U>(_x: T, _idx: u32, _val: U) -> T;
 /// `idx` must be in-bounds of the vector.
 #[rustc_intrinsic]
 #[rustc_nounwind]
-pub unsafe fn simd_extract<T, U>(_x: T, _idx: u32) -> U;
+pub const unsafe fn simd_extract<T, U>(_x: T, _idx: u32) -> U;
 
 /// Adds two simd vectors elementwise.
 ///

tests/codegen/issues/issue-84268.rs

@@ -1,10 +1,7 @@
 //@ compile-flags: -Copt-level=3 --crate-type=rlib
-#![feature(intrinsics, repr_simd)]
+#![feature(core_intrinsics, repr_simd)]
 
-extern "rust-intrinsic" {
-    fn simd_fabs<T>(x: T) -> T;
-    fn simd_eq<T, U>(x: T, y: T) -> U;
-}
+use std::intrinsics::simd::{simd_eq, simd_fabs};
 
 #[repr(simd)]
 pub struct V([f32; 4]);