Add CastKind::Transmute to MIR

Updates `interpret`, `codegen_ssa`, and `codegen_cranelift` to consume the new cast instead of the intrinsic.

Includes `CastTransmute` for custom MIR building, to be able to test the extra UB.

Author: scottmcm

core/src/intrinsics/mir.rs

@@ -329,6 +329,14 @@ define!(
     /// See [`Field`] for documentation.
     fn Variant<T>(place: T, index: u32) -> ()
 );
+define!(
+    "mir_cast_transmute",
+    /// Emits a `CastKind::Transmute` cast.
+    ///
+    /// Needed to test the UB when `sizeof(T) != sizeof(U)`, which can't be
+    /// generated via the normal `mem::transmute`.
+    fn CastTransmute<T, U>(operand: T) -> U
+);
 define!(
     "mir_make_place",
     #[doc(hidden)]