transmute should also assume non-null pointers

Previously it only did integer-ABI things, but this way it does data pointers too.  That gives more information in general to the backend, and allows slightly simplifying one of the helpers in slice iterators.

core/src/slice/iter/macros.rs

@@ -30,7 +30,7 @@ macro_rules! if_zst {
             $zst_body
         } else {
             // SAFETY: for non-ZSTs, the type invariant ensures it cannot be null
-            let $end = unsafe { *(&raw const $this.end_or_len).cast::<NonNull<T>>() };
+            let $end = unsafe { mem::transmute::<*const T, NonNull<T>>($this.end_or_len) };
             $other_body
         }
     }};