[GVN] Add tests for generic pointees with PtrMetadata

Author: scottmcm

Diff for: core/src/intrinsics/mir.rs

@@ -444,6 +444,13 @@ define!(
     /// generated via the normal `mem::transmute`.
     fn CastTransmute<T, U>(operand: T) -> U
 );
+define!(
+    "mir_cast_ptr_to_ptr",
+    /// Emits a `CastKind::PtrToPtr` cast.
+    ///
+    /// This allows bypassing normal validation to generate strange casts.
+    fn CastPtrToPtr<T, U>(operand: T) -> U
+);
 define!(
     "mir_make_place",
     #[doc(hidden)]