Rollup merge of #98441 - calebzulawski:simd_as, r=oli-obk

Implement simd_as for pointers

Expands `simd_as` (and `simd_cast`) to handle pointer-to-pointer, pointer-to-integer, and integer-to-pointer conversions.

cc ``@programmerjake`` ``@thomcc``

src/test/ui/simd/intrinsic/ptr-cast.rs

@@ -0,0 +1,33 @@
+// run-pass
+
+#![feature(repr_simd, platform_intrinsics)]
+
+extern "platform-intrinsic" {
+    fn simd_cast_ptr<T, U>(x: T) -> U;
+    fn simd_expose_addr<T, U>(x: T) -> U;
+    fn simd_from_exposed_addr<T, U>(x: T) -> U;
+}
+
+#[derive(Copy, Clone)]
+#[repr(simd)]
+struct V<T>([T; 2]);
+
+fn main() {
+    unsafe {
+        let mut foo = 4i8;
+        let ptr = &mut foo as *mut i8;
+
+        let ptrs = V::<*mut i8>([ptr, core::ptr::null_mut()]);
+
+        // change constness and type
+        let const_ptrs: V<*const u8> = simd_cast_ptr(ptrs);
+
+        let exposed_addr: V<usize> = simd_expose_addr(const_ptrs);
+
+        let from_exposed_addr: V<*mut i8> = simd_from_exposed_addr(exposed_addr);
+
+        assert!(const_ptrs.0 == [ptr as *const u8, core::ptr::null()]);
+        assert!(exposed_addr.0 == [ptr as usize, 0]);
+        assert!(from_exposed_addr.0 == ptrs.0);
+    }
+}