Avoid a cast in ptr::slice_from_raw_parts(_mut)

Casting to `*const ()` or `*mut ()` just bloats the MIR, so let's not.

If ACP#362 goes through we can keep calling `ptr::from_raw_parts(_mut)` in these also without the cast, but that hasn't had any libs-api attention yet, so I'm not waiting on it.

Author: scottmcm

Diff for: core/src/ptr/mod.rs

@@ -812,7 +812,7 @@ pub const fn from_mut<T: ?Sized>(r: &mut T) -> *mut T {
 #[rustc_allow_const_fn_unstable(ptr_metadata)]
 #[rustc_diagnostic_item = "ptr_slice_from_raw_parts"]
 pub const fn slice_from_raw_parts<T>(data: *const T, len: usize) -> *const [T] {
-    from_raw_parts(data.cast(), len)
+    intrinsics::aggregate_raw_ptr(data, len)
 }
 
 /// Forms a raw mutable slice from a pointer and a length.
@@ -858,7 +858,7 @@ pub const fn slice_from_raw_parts<T>(data: *const T, len: usize) -> *const [T] {
 #[rustc_const_unstable(feature = "const_slice_from_raw_parts_mut", issue = "67456")]
 #[rustc_diagnostic_item = "ptr_slice_from_raw_parts_mut"]
 pub const fn slice_from_raw_parts_mut<T>(data: *mut T, len: usize) -> *mut [T] {
-    from_raw_parts_mut(data.cast(), len)
+    intrinsics::aggregate_raw_ptr(data, len)
 }
 
 /// Swaps the values at two mutable locations of the same type, without