Fix metadata on PtrToPtr cast

[dkcumming]

PtrToPtr casts were mistakenly ignoring metadata ORIGIN_SIZE and setting it to noMetadataSize after the cast each time. This PR preserves the ORIGIN_SIZE across the cast.

More Details

This issue was discovered working with Iterators, in particular the relationship between std::slice::Iter<'_, T>::new and <std::slice::iter<'_, thing> as std::iter::Iterator>::next. When iterating through a collection there was no match at the end that should of terminated iterating. An Iter tracks position through the collection with 2 pointers:

pub struct Iter<'a, T: 'a> {
    /// The pointer to the next element to return, or the past-the-end location
    /// if the iterator is empty.
    ///
    /// This address will be used for all ZST elements, never changed.
    ptr: NonNull<T>,
    /// For non-ZSTs, the non-null pointer to the past-the-end element.
    ///
    /// For ZSTs, this is `ptr::without_provenance_mut(len)`.
    end_or_len: *const T,
    _marker: PhantomData<&'a T>,
}

In order to terminate ptr == end_or_len, however in KMIR this check was always failing due to the pointer representation:

Pointers in KMIR check equality on all metadata fields, so this equality would never pass. The correct ORIGIN_SIZE should be dynamicSize(2) as they are pointers to elements of a slice. The Iter is instantiated with the incorrect ORIGIN_SIZE.

The construction of an Iter by the new function does a series of pointer casts. The cast that strips the ORIGIN_SIZE in KMIR came from _5 = Cast-PtrToPtr mv(6) which is a cast of *mut Thing to *const Thing:

[mariaKt]

Looks good, thank you.